Stormwater Treatment Performance
Understand whether your proposed stormwater treatment measures achieve the required water-quality outcome.
For architects, developers, planners and project teams requiring quantitative assessment of rainwater tanks, raingardens, impervious catchments and other stormwater treatment measures.
Send Your Project DocumentsIn Brief
A STORM assessment is a quantitative stormwater treatment assessment used to test whether proposed measures are expected to achieve the required water-quality outcome for a development.
The assessment considers the stormwater catchments created by roofs, driveways, paving and other impervious surfaces, together with treatment measures such as rainwater tanks, raingardens, infiltration systems and permeable surfaces. It examines how much runoff is directed to treatment and how effectively the modelled treatment arrangement reduces stormwater pollutant impacts.
STORM is commonly used within Victorian planning applications where a council or planning authority requires quantitative evidence that stormwater treatment objectives have been addressed. It assesses water-quality performance and does not replace detailed hydraulic drainage design, flood modelling or on-site detention design where those services are required.
For a more detailed explanation of the assessment inputs, modelling process and report outcomes, read our detailed STORM assessment guide.
The treatment performance of proposed measures, including the impervious catchments connected to rainwater tanks, raingardens and other accepted stormwater systems.
It may be requested as part of a Victorian planning application where quantitative evidence of stormwater treatment performance is required.
The result indicates whether the modelled catchments and treatment measures are expected to satisfy the applicable stormwater quality objective.
Knowledge Navigation
Follow the assessment pathway from understanding what STORM measures through to model inputs, treatment measures, project information and interpretation of the final stormwater quality result.
Foundation
Understand what a STORM assessment measures and how it is used to test the quantitative stormwater treatment performance of a development.
Assessment Process
See how catchments, impervious surfaces, treatment measures and water-reuse assumptions are translated into a quantitative assessment result.
Planning Requirement
Explore when a council or planning authority may request quantitative STORM modelling as part of a Victorian development application.
Treatment Measures
Review common modelled measures including rainwater tanks, raingardens, infiltration systems and permeable surfaces.
Project Information
Identify the plans, catchment areas, tank details, reuse connections and treatment-system information normally needed to complete the assessment.
Assessment Outcome
Understand what the STORM result indicates, what happens when the model falls short and which inputs may improve treatment performance.
Assessment Guidance
Service Boundaries
STORM Fundamentals
A STORM assessment connects runoff-generating surfaces with proposed treatment measures to determine whether the development achieves the applicable stormwater-quality objective.
STORM stands for Stormwater Treatment Objective – Relative Measure. The assessment compares the proposed treatment arrangement with an established stormwater-quality objective and produces a quantitative result indicating whether the required level of performance has been achieved.
The model considers the areas of a development that generate runoff, including roofs, driveways, paved areas and other impervious surfaces. It then examines how those catchments are connected to rainwater tanks, raingardens, infiltration systems, permeable surfaces or other recognised treatment measures.
The assessment does more than confirm that treatment measures appear on the drawings. It tests whether the nominated measures are appropriately sized, connected and supported by credible water-reuse assumptions so that the modelled development can achieve the relevant treatment target.
Current terminology: Melbourne Water has replaced the original STORM Calculator with BlueFactor. However, STORM remains a familiar term in existing permits, council documentation and project discussions. The appropriate assessment tool and reporting format should therefore be confirmed against the requirements applying to the individual project.
Assessment Input
Roof, driveway, paving and other impervious areas are identified according to the runoff they generate.
Modelled Measures
The assessment records which catchments connect to each tank, raingarden, permeable surface or other treatment measure.
Assessment Outcome
The resulting output indicates whether the modelled catchments and measures achieve the applicable stormwater-quality objective.
Scope Boundary
The assessment evaluates stormwater-quality treatment performance. It does not replace detailed hydraulic calculations, pipe and pit sizing, flood modelling, legal point of discharge advice or on-site detention design where those services are required.
Assessment Process
The assessment translates runoff-generating surfaces, treatment connections and water-reuse assumptions into a quantitative stormwater treatment result.
Step 01
The project location, development type, planning request and available documentation are reviewed first. This confirms the applicable assessment pathway, the required treatment objective and the information that must be demonstrated.
Step 02
The development is divided into relevant runoff-generating areas, including roofs, driveways, paved courtyards, car parks and other impervious surfaces.
Step 03
Each catchment is connected to the relevant rainwater tank, raingarden, infiltration system, permeable surface or other recognised treatment measure included in the design.
Step 04
Catchment areas, tank storage capacity, water-reuse connections, treatment dimensions and other relevant project assumptions are entered into the assessment model.
Step 05
The model produces a quantitative result indicating whether the combined catchments and treatment measures achieve the applicable stormwater-quality objective.
Step 06
Where the initial result falls short, practical changes can be tested. These may include increasing the connected catchment, improving tank reuse, resizing a treatment measure or adding another accepted treatment element.
The final assessment should align with the architectural, landscape and stormwater documentation submitted for planning. Each modelled treatment measure should appear clearly on the drawings, with consistent dimensions, catchment connections and water-reuse assumptions.
Completing the assessment before the design is fully documented allows treatment measures to be tested while their sizes, locations and connections can still be coordinated. This can reduce late design changes and improve consistency across the project documentation.
For a closer explanation of how catchments, treatment measures and reuse assumptions influence the assessment outcome, read How Is a STORM Score Calculated?
Key Principle
Changes to roof areas, paving, tank capacity, treatment dimensions or reuse connections can alter the assessment result. The model may therefore need to be updated when relevant design information changes during planning, documentation or construction.
Planning Requirement
A quantitative stormwater treatment assessment may be needed where a Victorian development must demonstrate that runoff from its impervious surfaces will receive an appropriate level of treatment.
The requirement is determined by the planning controls applying to the site, local council policy, the development proposal and the information requested during the planning process. It may arise through an application checklist, council correspondence, a request for further information or a condition of planning permit.
The assessment may be submitted as a focused stormwater treatment report or used as supporting evidence within broader planning or sustainability documentation. Its role remains limited to demonstrating the quantitative treatment performance of the modelled catchments and measures.
Melbourne Water has replaced the original STORM Calculator with BlueFactor. Project correspondence may therefore refer to a STORM assessment, BlueFactor assessment or another accepted form of stormwater treatment modelling. The required tool and reporting format should be confirmed against the current council request and planning pathway.
For a more detailed explanation of the Victorian planning triggers and current assessment pathways, read When Is a STORM Assessment Required in Victoria?
Common Planning Triggers
The planning scheme, council checklist or project brief may require quantitative evidence of stormwater treatment performance.
New roofs, driveways, car parks or paved areas may increase the need to demonstrate how the additional runoff will be treated.
Council may request modelling where the submitted plans do not provide enough evidence that the proposed treatment measures achieve the required outcome.
A permit condition may require treatment modelling or revised stormwater documentation before plans can be endorsed or the project can progress.
Scope Check
A relatively straightforward development may be suited to a focused BlueFactor or STORM-style assessment. A larger or more complex project may instead require MUSIC modelling, broader stormwater documentation or separate civil and hydraulic input.
The project address, planning controls, council correspondence and current drawings should be reviewed before the assessment scope is confirmed. You can also review the common project types that may use a STORM-style assessment.
Treatment Measures
The assessment tests how nominated stormwater treatment measures interact with the impervious catchments created by the development and whether their combined performance achieves the applicable water-quality objective.
A treatment measure does not contribute to the result simply because it appears on the drawings. Its modelled performance depends on the runoff directed to it, its nominated size or capacity and any operational assumptions associated with filtration, infiltration or water reuse.
A development may rely on one measure or combine several measures to treat different catchments. The arrangement used in the assessment should reflect what can realistically be constructed, connected and maintained within the proposed design.
The measures and inputs available can vary according to the accepted assessment method and the requirements of the responsible authority. Each nominated measure should therefore be confirmed against the current project pathway and documented consistently across the relevant drawings and reports.
Common Modelled Measures
Capture and Reuse
Rainwater tanks collect runoff from connected roof catchments. Their treatment contribution depends on the relationship between the available storage, the amount of roof water received and the frequency with which stored water is reused.
Relevant inputs may include:
Read more in Can Rainwater Tanks Improve a STORM Rating?
Filtration
Raingardens and bioretention systems receive runoff from nominated catchments and filter it through a designed treatment area before the water is discharged, infiltrated or conveyed onward.
The model may consider the connected catchment and treatment area. The physical design must also coordinate inlets, overflows, levels, filter media, planting and long-term maintenance outside the assessment model.
Permeable Treatment
Permeable paving and related systems allow water to move through the finished surface rather than generating runoff in the same way as conventional impermeable pavement.
Any modelled contribution should reflect the proposed system, the area it serves and its acceptance within the assessment method applying to the project.
Site-Based Treatment
Where suitable and accepted, infiltration measures may allow treated stormwater to enter the surrounding soil rather than being discharged directly from the site.
Their feasibility can depend on soil conditions, groundwater, site levels, building proximity and drainage constraints. Geotechnical, civil or hydraulic verification may therefore be required separately.
Catchment Relationship
Treatment performance depends on both the measure and the stormwater catchment directed to it. A large rainwater tank connected to only a small roof area may provide limited additional benefit. A measure receiving runoff from a larger catchment may require greater capacity, more treatment area or more regular reuse.
Untreated driveways, courtyards, car parks and other impervious surfaces can also limit the overall result. Improving performance may therefore involve treating additional catchments rather than repeatedly increasing the size of one isolated measure.
See which model inputs may assist in improving the stormwater treatment result.
Practical Limitation
A measure should not be added solely to improve the numerical result. It must suit the available space, site conditions, drainage arrangement, building layout and maintenance responsibilities. Where specialist hydraulic, civil, landscape or geotechnical design is required, that work remains separate from the treatment assessment.
Project Information
A reliable assessment depends on current plans, clearly defined catchments and treatment details that accurately reflect the proposed development.
The assessor needs enough information to identify the area being assessed, determine which surfaces generate stormwater runoff, understand where that runoff is directed and model how the proposed treatment measures are expected to operate.
The required documents vary with project scale and complexity. A relatively small proposal may be assessed from a coordinated set of architectural plans and clear rainwater tank information. A larger development may also require landscape plans, civil information, drainage intent and more detailed schedules of catchment areas and treatment measures.
Preliminary assumptions may sometimes be used where parts of the design are still being developed. Those assumptions should be clearly identified, physically achievable and confirmed before the final assessment is issued for planning or approval.
Project Basics
Design Documentation
Catchment Inputs
Treatment Inputs
The assessment should represent the same modelled treatment arrangement shown throughout the project documentation. Tank capacities, catchment connections, reuse commitments and treatment dimensions should remain consistent across the relevant drawings, schedules and reports.
For example, a rainwater tank included in the model should appear on the plans, and its nominated reuse connections should be reflected in the architectural or services documentation. A modelled raingarden should occupy a real and buildable area without conflicting with access, utilities, retaining walls or required landscape elements.
Where architectural, landscape, civil and services information is being prepared by different consultants, early coordination can reduce conflicting assumptions and avoid unnecessary assessment revisions.
For a complete project-information checklist, read What Information Is Needed for a STORM Assessment?
Outdated drawings can produce an assessment that no longer represents the project. Where roof layouts, external surfaces, tank capacities, catchment connections, treatment measures or reuse commitments change, the model should be reviewed and updated where necessary.
Assessment Outcome
The result indicates how the modelled catchments and treatment measures perform against the applicable stormwater-quality benchmark.
The assessment combines the development’s runoff-generating surfaces, treatment connections and water-reuse assumptions to produce a quantitative performance result. This allows the project team and responsible authority to understand whether the modelled treatment arrangement is expected to achieve the required water-quality outcome.
Where the result reaches the applicable benchmark, the entered combination of catchments and treatment measures provides the required modelled performance. Where it falls below the benchmark, one or more inputs or treatment connections may need to be reviewed.
A 100% STORM rating is a relative measure of performance against the assessment benchmark. It does not mean that every litre of rainfall is retained, that every pollutant is removed or that every impervious surface is treated individually.
The result must also be read together with the assumptions entered into the model. Its reliability depends on the catchment areas, treatment dimensions, tank capacity and reuse arrangements accurately reflecting the proposed development.
For a detailed explanation of the commonly referenced benchmark, read What Does a 100% STORM Rating Mean?
Reading the Outcome
Benchmark Achieved
The modelled catchments and treatment measures collectively achieve the applicable stormwater-quality objective.
Below Benchmark
The current arrangement does not yet provide sufficient modelled treatment performance and may require practical refinement.
Design Revision
Changes to roofs, paving, tanks, catchment connections, reuse commitments or treatment dimensions can alter the result.
Below-Benchmark Results
A development can include rainwater tanks, raingardens or other measures and still remain below the required benchmark. Performance depends on how much runoff is generated, which catchments receive treatment and whether the nominated measures have sufficient capacity or credible reuse demand.
Roofs, driveways, paving or car parks may generate runoff without being connected to an accepted treatment measure.
A tank or raingarden may be included but receive too little runoff to make the required contribution to the overall result.
A rainwater tank may provide limited modelled benefit where there is insufficient regular demand to create capacity for future rainfall.
The nominated tank, raingarden or other measure may be too small for the catchment and runoff directed to it.
Read Common Reasons a Project Fails a STORM Assessment, or continue to how the modelled treatment result may be improved.
Scope Boundary
Reaching the assessment benchmark demonstrates the modelled water-quality performance of the nominated catchments and treatment measures. It does not confirm hydraulic capacity, on-site detention volume, flood behaviour, legal discharge arrangements or detailed civil design.
Those matters may still require separate documentation, specialist design and approval from the relevant council, drainage authority or engineering consultant.
Model Refinement
Where the initial result falls below the required benchmark, targeted changes to catchments, treatment capacity or water-reuse assumptions may improve modelled performance.
The appropriate adjustment depends on why the model remains below the required stormwater-quality objective. The issue may be an undersized measure, insufficient connected catchment, limited rainwater reuse or a substantial area of untreated roof, driveway or paving.
Improving the result does not necessarily require another treatment system. Reallocating roof catchments, coordinating drainage connections or making more effective use of an existing rainwater tank may provide a more practical improvement than adding an isolated measure.
Any revised input must reflect a solution that can be documented, constructed and maintained. The model should not rely on catchment connections, tank uses or treatment dimensions that are not supported by the project drawings.
Potential Model Adjustments
Catchment Connection
Directing additional roof runoff to an accepted treatment measure may improve the result, provided the proposed drainage route is practical and shown consistently on the drawings.
Storage Capacity
A larger tank may improve performance where the existing usable storage is insufficient for the roof catchment and realistic water demand connected to it.
Water Reuse
Suitable toilet, laundry or irrigation connections can draw down stored rainwater and create capacity for future rainfall. Only realistic and documented reuse commitments should be entered.
Treatment Area
A treatment area may be added or resized where adequate space, levels and catchment connections are available and the physical design can be properly coordinated.
Untreated Catchments
Runoff from driveways, courtyards, car parks or other impervious areas may be redirected to an accepted measure where the site layout and drainage design permit.
Impervious Area
Reducing avoidable impervious area or using an accepted permeable treatment may reduce the volume of runoff that remains untreated within the assessment.
Assessment Timing
Treatment inputs are easier to coordinate while roof drainage, external surfaces, tank locations and treatment areas are still being developed. At this stage, catchments and measures can be adjusted before the project documentation becomes difficult to revise.
Where modelling is completed late, the project may have limited space for treatment areas, established drainage routes or fixed tank locations. This can narrow the available options and lead to more disruptive changes.
Review the treatment measures that may be modelled or return to how the assessment result is interpreted.
Performance Principle
The aim is to achieve the applicable stormwater-quality objective using inputs and treatment measures that reflect the actual development. A higher numerical result is not useful where the assumed catchments, reuse connections or treatment dimensions cannot be delivered and maintained.
Service Boundary
STORM quantifies the treatment performance of selected catchments and measures, while WSUD develops the broader water-sensitive response for the site.
STORM and Water Sensitive Urban Design are closely connected, but they answer different project questions. A STORM assessment tests whether the modelled catchments and treatment measures achieve the applicable stormwater-quality objective. A WSUD response considers how water should be managed and integrated across the development more broadly.
The STORM assessment remains focused on impervious surfaces, treatment connections, water-reuse assumptions and the resulting quantitative performance. WSUD may consider these matters alongside site permeability, landscape integration, infiltration, water conservation, maintenance and the relationship between stormwater and the wider development layout.
Depending on the planning requirement, a project may need a focused treatment assessment, a broader WSUD report or coordinated documentation containing both elements. The requested scope should be confirmed from the council correspondence and applicable planning pathway.
STORM Assessment
STORM asks whether the modelled combination of runoff-generating surfaces and treatment measures achieves the required water-quality performance.
WSUD Response
WSUD asks how stormwater, water reuse, infiltration, landscape and site planning should work together across the development.
A WSUD response may establish where runoff will be captured, reused, filtered or integrated into the landscape. The treatment assessment can then model the relevant catchment areas, tank inputs, reuse assumptions and treatment dimensions to test whether the required quantitative outcome is achieved.
Where both are required, the nominated measures and project commitments should remain consistent across the assessment, WSUD documentation and submitted drawings.
For a more detailed comparison of their separate roles and reporting boundaries, read STORM vs WSUD: What Is the Difference?
Canonical Boundary
The STORM assessment should remain focused on quantitative treatment performance. The wider planning, landscape and water-management strategy belongs to the WSUD response.
Explore the WSUD Knowledge Hub
Service Boundary
STORM tests stormwater-quality treatment performance, while On-Site Detention manages the temporary storage and controlled release of runoff.
STORM and On-Site Detention may both form part of a development’s stormwater response, but they address different performance outcomes. A STORM assessment tests whether the modelled catchments and treatment measures achieve the applicable water-quality objective. OSD addresses how runoff is temporarily stored and released from the site at a controlled rate.
A development may therefore achieve its stormwater treatment benchmark and still require a separate detention system. Likewise, an OSD design may control peak discharge without demonstrating that the proposed development achieves the required water-quality treatment outcome.
Where both requirements apply, the physical systems should be coordinated. Tanks, pits, treatment measures, overflow connections and outlet controls may interact within the same drainage arrangement even though their assessment purposes remain separate.
STORM Assessment
STORM asks whether the modelled catchments and treatment measures provide the required stormwater-quality performance.
On-Site Detention
OSD asks how runoff can be temporarily detained and released at an acceptable rate to manage downstream drainage impacts.
A rainwater tank may sometimes contribute to both treatment and detention, but the storage allocated to each function must be clearly defined. Reuse storage, detention storage, overflow levels and controlled outlets perform different roles and should not be treated as interchangeable.
Within the treatment assessment, performance may depend on the connected roof catchment, usable storage and regular water demand. For OSD, the relevant considerations include the available detention volume, outlet configuration and controlled discharge during the nominated storm event.
Where a combined tank arrangement is proposed, the treatment inputs should remain consistent with the hydraulic or civil design so that each claimed outcome is supported by the project documentation.
Canonical Boundary
The STORM assessment should remain focused on the quantitative water-quality performance of the modelled treatment measures. Detention volume, peak flow and controlled discharge belong to the civil or hydraulic design scope.
Continue to the distinction between STORM and hydraulic engineering, or return to how the treatment result is interpreted.
Service Boundary
STORM may provide quantitative evidence for the stormwater section of a submission, while an SMP or SDA documents the development’s broader sustainability response.
A Sustainable Management Plan or Sustainable Design Assessment explains how a proposed development responds to the wider sustainability requirements applying through the planning pathway. Depending on the project and council request, this may include several environmental and design categories rather than stormwater treatment alone.
STORM has a more focused role. It models the relationship between runoff-generating surfaces, treatment measures and water-reuse assumptions to determine whether the applicable stormwater-quality objective is achieved.
Where both are required, the quantitative treatment result may be referenced within the water or stormwater section of the broader report. The catchments, tank capacities, reuse connections and treatment dimensions should remain consistent across the assessment, sustainability documentation and submitted drawings.
STORM Assessment
STORM answers a defined technical question about the modelled stormwater treatment performance of the development.
SMP or SDA
An SMP or SDA coordinates the wider sustainability commitments required for the planning application.
The broader report may summarise the proposed stormwater treatment measures and reference the quantitative assessment as evidence that the applicable water-quality objective has been addressed.
Any tank capacity, connected catchment, reuse commitment or treatment area described in the SMP or SDA should match the information used in the assessment. The same measures should also be identifiable within the relevant architectural, landscape or civil documentation.
The STORM result remains supporting technical evidence. It does not replace the other sustainability categories, explanations and project commitments that belong within the SMP or SDA.
Canonical Boundary
The STORM assessment should remain focused on quantitative treatment performance. The wider planning sustainability narrative and project-wide commitments belong within the SMP or SDA.
Explore the SMP & SDA Knowledge HubService Boundary
STORM may provide focused stormwater-treatment evidence, while BESS assesses the proposed development across a broader sustainability framework.
BESS, or the Built Environment Sustainability Scorecard, is used within Victorian planning pathways to assess a proposed development across multiple sustainability categories. Stormwater forms one part of that wider assessment rather than determining the complete BESS outcome.
A STORM assessment has a narrower technical role. It models runoff-generating surfaces, treatment connections and relevant water-reuse assumptions to determine whether the proposed measures achieve the applicable stormwater-quality objective.
Where accepted for the project, the resulting treatment evidence may support the stormwater component of the BESS submission. Achieving the required stormwater result does not establish that the development has achieved its wider BESS requirements.
Current BESS documentation refers to both Blue Factor assessments and STORM ratings. The assessment method, evidence and reporting format should therefore be confirmed against the current BESS pathway and council requirements applying to the project.
STORM Assessment
STORM answers a defined technical question about the water-quality performance of the modelled catchments and treatment measures.
BESS Assessment
BESS brings together multiple sustainability categories and project commitments within the Victorian planning submission.
Where the stormwater result is used within BESS, both assessments should rely on the same project information. Roof catchments, tank capacities, reuse connections, permeable areas and treatment measures should remain consistent across the models and submitted drawings.
A rainwater tank may influence both stormwater treatment and wider water-related commitments. Its stated capacity, connected roof area and intended reuse should therefore describe one coordinated and physically achievable system.
Where relevant design information changes, the stormwater assessment and associated BESS inputs should be reviewed so that the planning documentation continues to represent the same proposal.
Canonical Boundary
The stormwater assessment should remain focused on quantitative treatment performance. Wider sustainability scoring, category requirements and project commitments belong within the BESS assessment.
Explore the BESS Knowledge Hub
Service Boundary
A STORM assessment evaluates stormwater-quality treatment performance. It does not replace the civil or hydraulic design of the site drainage system.
STORM and hydraulic engineering may both refer to roofs, paved areas, tanks, pits and other stormwater infrastructure, but they use that information for different purposes. STORM tests whether the modelled catchments and treatment measures achieve the applicable water-quality objective.
Civil or hydraulic engineering determines how runoff is collected, conveyed, temporarily stored and discharged through the physical drainage system. It may address pipe and pit capacity, site levels, overflow routes, detention requirements and permitted discharge arrangements.
A development can therefore achieve its required treatment result before detailed drainage design is complete. That result does not confirm that the drainage network is hydraulically adequate or that all authority requirements have been satisfied.
STORM Assessment
STORM asks whether the modelled treatment arrangement provides the required water-quality outcome.
Hydraulic Engineering
Hydraulic engineering asks how stormwater will move safely through and away from the development.
Outside the Assessment Scope
Reaching the required treatment benchmark should not be interpreted as approval of the complete drainage system. The following matters remain outside the treatment assessment.
The assessment does not size drainage infrastructure or verify whether pipes and pits can convey the required design flows.
STORM does not establish flood levels, major storm behaviour or detailed overland flow paths across the development.
It does not calculate required detention storage, outlet controls or permissible discharge rates for an OSD system.
The treatment model does not determine or approve where runoff may legally connect to council or other drainage infrastructure.
Document Coordination
Although their technical purposes remain separate, the modelled treatment arrangement must work within the physical drainage design. A roof catchment assigned to a rainwater tank must be capable of draining to that tank, while a paved catchment assigned to a raingarden must have a practical flow path to the treatment area.
Site levels, available falls, overflow arrangements and detention requirements can affect whether the nominated measures can operate as assessed. Early coordination reduces the risk of the model relying on connections that become impractical during detailed design.
Where engineering changes alter the catchments, tank configuration, treatment dimensions or drainage connections, the treatment assessment should be reviewed so that its reported result continues to represent the development.
Canonical Boundary
The STORM assessment should remain focused on quantitative water-quality treatment performance. Hydraulic calculations, drainage capacity, flood behaviour, detention design and civil documentation belong within the engineering scope.
Review the separate boundary between STORM and On-Site Detention, or return to how the treatment result should be interpreted.
Planning Submission
The assessment provides quantitative evidence that the modelled stormwater treatment measures are expected to achieve the applicable water-quality objective.
Within a Victorian planning application, the assessment helps the responsible authority understand how runoff-generating surfaces are connected to rainwater tanks, raingardens and other treatment measures, and whether their combined modelled performance reaches the required benchmark.
The assessment may be submitted as focused stormwater treatment evidence or incorporated into broader planning documentation. Its role remains limited to the quantitative performance of the nominated catchments, treatment connections and water-reuse assumptions.
The submission should allow the authority to identify what has been modelled, where the treatment measures are located and how the assumptions used in the assessment are reflected in the proposed development.
Submission Evidence
Assessment Output
The documentation should identify the assessment result, the treatment measures entered and the principal assumptions used to demonstrate the required water-quality performance.
Project Drawings
Rainwater tanks, raingardens, permeable areas and other modelled measures should be identifiable on the relevant architectural, landscape or civil drawings.
Model Consistency
Connected catchments, tank capacities, treatment areas and reuse commitments should match the information represented in the assessment.
Planning Requirement
The submitted evidence should respond directly to the planning checklist, request for further information, permit condition or other authority requirement that triggered the assessment.
Document Coordination
A clear planning package should show which roofs, driveways, paved areas or other catchments are directed to each treatment measure. The location and size of those measures should be understandable without relying on undocumented assumptions.
Where rainwater reuse contributes to the result, the nominated toilet, laundry or irrigation connections should also appear within the relevant drawings, schedules or services information.
Review the project information normally needed for the assessment.
Assessment Timing
The assessment is most useful while roof drainage, tank capacity, external surfaces and treatment areas can still be coordinated. This allows model inputs to be tested before they become fixed within the planning documentation.
Where modelling is completed after the drawings are substantially resolved, there may be less flexibility to increase tank capacity, redirect catchments or provide additional treatment area if the initial result falls below the required benchmark.
See which model inputs may improve the treatment result, or review when the assessment requirement may arise.
Scope Boundary
The assessment addresses the quantitative water-quality performance of the nominated catchments and treatment measures. Planning approval may still depend on separate drainage, detention, flooding, landscape, civil engineering and authority requirements.
Project Suitability
The assessment pathway is generally most suitable for smaller Victorian developments with clearly defined catchments and relatively straightforward treatment measures.
Project type alone does not determine whether a stormwater treatment assessment is required. The planning controls, responsible authority, site area, development layout and complexity of the proposed treatment arrangement must also be considered.
The original STORM Calculator has now been replaced by BlueFactor. The current small-scale pathway is generally intended for developments under one hectare that do not include public roads. Project correspondence may nevertheless continue to refer to a STORM assessment or STORM rating.
Within those limits, the assessment can be applied where runoff-generating surfaces and their treatment connections can be represented clearly. Larger sites, public-road infrastructure or complex treatment trains may require MUSIC or another authority-approved modelling approach.
Common Applications
Small Residential
New homes, additions and associated external works may be suitable where the roof, driveway and paved catchments can be represented through a straightforward treatment model.
Medium-Density Residential
Smaller multi-dwelling projects may be suitable where individual roofs, shared driveways, courtyards and treatment measures can be divided into clearly defined catchments.
Small Commercial
Smaller offices, retail premises and similar developments may use the pathway where roof, parking and hardstand runoff can be assigned to straightforward treatment measures.
Assessment Fit
A project is more likely to suit the simplified assessment pathway where the site boundary, runoff-generating surfaces and treatment connections can be represented without extensive hydraulic or temporal modelling.
The site or development area being assessed can be identified clearly and measured from the available project drawings.
Roof, driveway, paving and other impervious areas can be allocated to specific treatment measures without an overly complex drainage network.
The proposal uses measures that can be represented within the accepted assessment method, such as tanks, raingardens or suitable permeable treatments.
The development is generally below one hectare, does not include public roads and does not rely on a complex sequence of treatment systems.
Alternative Pathway
A more detailed pathway may be appropriate where the development contains numerous subcatchments, public roads, staged infrastructure, wetlands, major public-realm works or a sequence of connected treatment measures.
MUSIC can represent catchments, drainage connections and treatment trains in greater detail. The responsible authority should confirm the required model before substantial assessment work begins.
Review the difference between STORM-style assessment and MUSIC modelling, or return to how the planning requirement is determined.
Scope Check
Two apparently similar developments may require different assessment methods because of their site area, council requirements, road interfaces, catchment complexity or proposed treatment systems. The current planning request and project documentation should always be reviewed before the assessment pathway is confirmed.
Modelling Boundary
A STORM-style or BlueFactor assessment provides a simplified treatment result for suitable smaller developments, while MUSIC allows more detailed modelling of catchments, drainage connections and treatment trains.
Both pathways can help demonstrate how proposed stormwater measures respond to runoff generated by a development. The appropriate method depends on the site scale, catchment complexity, treatment arrangement, project risk and the requirements of the responsible authority.
The original STORM Calculator has been replaced by BlueFactor. However, existing permits, council requests and project discussions may continue to use the term STORM. Within this Knowledge Hub, a STORM-style assessment refers to the simplified score-based pathway used for relatively small and straightforward developments.
MUSIC provides a more detailed conceptual model of the project or catchment. It can represent multiple subcatchments, drainage links and a sequence of connected treatment measures where the simplified pathway does not provide enough detail.
STORM or BlueFactor
This pathway is generally used where the development and its treatment connections can be represented through a relatively accessible and simplified assessment.
MUSIC
MUSIC can represent how runoff moves between multiple catchments and treatment measures where more detailed system modelling is required.
More Detailed Modelling
MUSIC may be more appropriate where the development cannot be represented adequately through a simplified score-based assessment or where the authority specifically requests a more detailed model.
Runoff may pass through several treatment measures in sequence before reuse, discharge or entry into another part of the system.
The development may contain several buildings, precincts, stages or drainage areas requiring separate catchment representation.
Wetlands, sediment ponds, major bioretention systems or other substantial assets may require more detailed modelling of system performance.
Council or another responsible authority may nominate MUSIC or an equivalent model as the required evidence for the project.
Pathway Selection
The correct pathway should be confirmed from the planning controls, council correspondence, site area, development type and proposed treatment arrangement. The simplest available tool is not necessarily the method that the authority will accept.
A smaller low-risk development with clearly defined catchments may suit the STORM-style or BlueFactor pathway. A larger development, complex drainage network or multi-stage treatment system may require MUSIC or another approved method.
Review which projects are commonly suited to the simplified pathway, or return to how the assessment requirement is determined.
Canonical Boundary
The STORM-style or BlueFactor pathway should remain focused on simplified treatment verification for suitable projects. Detailed catchment simulation, drainage links and treatment-train modelling belong within the MUSIC pathway.
Time and Cost
There is no single turnaround time or fixed fee for every project. Both are confirmed after the current plans, assessment pathway and proposed treatment measures have been reviewed.
A smaller development with clearly defined catchments, current drawings and a straightforward rainwater tank or treatment arrangement can usually be scoped more directly than a project containing several buildings, shared surfaces or multiple treatment measures.
The initial assessment result can also affect the programme and fee. Where the nominated measures achieve the required benchmark, the assessment may proceed to final documentation without extensive revision. Where the result falls short, further modelling and design coordination may be required.
Reviewing the latest drawings and authority correspondence before work begins allows the assessment scope, expected turnaround and professional fee to be established more reliably.
Assessment Timing
Professional Fee
Efficient Assessment
The site, roof, paving, landscape and treatment information should represent the latest development proposal.
Tank capacities, connected catchments, reuse commitments and treatment dimensions should be clearly identified.
Council correspondence or permit conditions should identify the required assessment pathway and expected evidence where available.
Potential Delays
Missing catchment areas, unclear tank connections or unresolved treatment dimensions can prevent the model from representing the proposed development reliably. Further information may need to be requested before the assessment can be completed.
Conflicting documents can also create additional work. For example, a tank capacity shown on the architectural plans may differ from the value recorded elsewhere, or a modelled raingarden may not yet appear on the landscape drawings.
Providing coordinated information at the beginning helps reduce clarification requests, remodelling and avoidable revisions.
Scope Review
A scope review should include the project address, current drawings, proposed treatment information and any council request, RFI item or permit condition relevant to stormwater treatment.
Review the complete list of information normally needed for a STORM assessment. Where the initial result may require revision, also see which model inputs can improve treatment performance.
Pricing Principle
The quotation should reflect the actual catchment complexity, treatment inputs, reporting requirements and likely level of coordination. Reviewing the project first helps distinguish a focused modelling task from work that may involve unresolved design information or several assessment iterations.
Frequently Asked Questions
Concise answers about STORM terminology, treatment modelling, planning evidence and the transition to BlueFactor.
STORM stands for Stormwater Treatment Objective – Relative Measure. It refers to a simplified method for comparing the modelled stormwater-treatment performance of a development against an applicable water-quality objective.
Yes. Melbourne Water has replaced the original STORM Calculator with BlueFactor for current small-development assessments. Existing permits, council correspondence and older planning documents may still refer to a STORM assessment or STORM rating.
A STORM-style assessment models runoff-generating surfaces, treatment connections and relevant water-reuse assumptions to determine whether the proposed measures achieve the required treatment outcome. Read the full explanation in What Is a STORM Assessment?
No. The requirement depends on the planning controls, responsible authority, development proposal and evidence requested for the application. Review when an assessment may be required.
It assesses the modelled water-quality performance of the nominated catchments and treatment measures. The result indicates whether their combined treatment response reaches the applicable benchmark rather than approving the complete drainage system.
Typical inputs include the project address, current site and roof plans, impervious surface areas, tank capacities, reuse connections, treatment dimensions and relevant authority correspondence. See the complete project information checklist.
The documentation may identify the assessment result, modelled catchments, treatment measures, reuse assumptions and information showing how those inputs relate to the project drawings. The exact submission format depends on the authority request.
Yes, where the tank receives a suitable roof catchment and stored water is reused regularly. A larger tank does not automatically produce a better result if the connected catchment or credible reuse demand is limited.
Yes, where the measure can be represented within the applicable assessment pathway and is supported by a defined catchment, appropriate dimensions and coordinated project drawings. Review the treatment measures that may be modelled.
The catchment allocation, treatment capacity or reuse assumptions may need to be refined. Possible adjustments are explained in Which Inputs Can Improve a STORM Result?
No. The assessment does not size pipes or pits, determine detention storage, model flooding or approve discharge arrangements. Those matters belong within the relevant civil or hydraulic engineering scope.
No. STORM focuses on quantitative treatment verification, while WSUD develops the broader water-sensitive response across the site. See how STORM and WSUD relate.
No. STORM addresses stormwater-quality treatment, while On-Site Detention addresses temporary storage and controlled discharge. A development may require both outcomes to be demonstrated separately.
The STORM-style or BlueFactor pathway provides simplified treatment verification for suitable smaller developments. MUSIC can model multiple subcatchments, drainage links and connected treatment trains in greater detail. Review how the modelling pathway is selected.
Treatment evidence may support the stormwater section of broader planning documentation. Current BESS pathways may require BlueFactor rather than the former STORM tool, so the applicable BESS version and council requirement should be confirmed.
Potentially. Changes to catchment areas, paving, tank capacities, reuse connections, treatment dimensions or drainage allocation can alter the result. Material design changes should be reviewed before relying on the existing assessment.
Timing and cost depend on the project scale, catchment complexity, treatment measures, available documentation and number of modelling iterations. Review the factors affecting assessment time and cost.
Related Planning Pathways
A STORM assessment remains focused on quantitative stormwater-treatment performance. These related pathways address the wider site response and planning documentation that may surround that assessment.
Integrated Water Strategy
Explore the broader site-wide approach to stormwater, water reuse, infiltration, permeability, landscape integration and long-term water management.
Explore the WSUD Knowledge Hub →
Planning Sustainability
Understand how stormwater-treatment evidence may support the water section of a broader Sustainable Management Plan or Sustainable Design Assessment.
Explore the SMP & SDA Knowledge Hub →
Victorian Planning
See how current stormwater evidence sits within a wider Victorian planning assessment covering water, energy, indoor environment quality and other sustainability categories.
Explore the BESS Knowledge Hub →
Project Review
Send the available site plan, architectural drawings, proposed rainwater tank information, treatment-system details and relevant council requirements for an initial review. Certified Energy can help determine whether a STORM assessment is the appropriate pathway for the project.
Early assessment can help test whether the proposed catchments, rainwater reuse and treatment measures achieve the required water-quality outcome before the planning documentation becomes difficult to revise.
Last reviewed: June 2026. This page is maintained by Certified Energy as part of its Residential Performance Knowledge Hub.
