A 100% STORM rating generally means that the modelled stormwater treatment strategy reaches the applicable best-practice water-quality benchmark for the development.
A 100% STORM rating indicates that the combined treatment measures entered into the assessment provide enough modelled performance to meet the reference stormwater-quality objectives.
The result may be achieved through a combination of rainwater tanks, connected roof catchments, regular rainwater reuse, raingardens, permeable surfaces and other suitable treatment measures.
It does not mean that 100% of rainfall is retained, 100% of pollutants are removed or every impervious surface is individually treated.
It also does not confirm that drainage design, detention, flood behaviour, discharge arrangements or every planning requirement has been resolved. The rating applies specifically to the modelled stormwater treatment performance.
The STORM rating is a relative measure rather than a literal percentage of water or pollution removed from the site.
The assessment compares the proposed treatment performance with established best-practice stormwater-quality objectives. A result of 100% indicates that the modelled design has reached the benchmark represented by the assessment tool.
Those best-practice objectives commonly address reductions in:
The treatment measures are assessed together across the proposed development. The resulting rating expresses how the complete strategy performs relative to those objectives.
For a broader explanation of the assessment process, visit the Certified Energy STORM Knowledge Hub.
Victorian stormwater treatment requirements use quantitative objectives to define best-practice water-quality performance.
These objectives commonly require the development’s treatment strategy to achieve:
A 100% rating indicates that the proposed combination of catchments and treatment measures achieves the reference level represented by these objectives.
It does not mean the project removes 100% of suspended solids, phosphorus, nitrogen or litter.
The word “100%” describes performance against the benchmark, not total pollutant removal.
A development can reach the benchmark through one treatment measure or, more commonly, a coordinated combination of measures.
These may include:
The result depends on the relationship between the runoff-generating surfaces and the measures that receive and treat that runoff.
Simply including a tank or raingarden on the plans does not guarantee a 100% rating. Its capacity, connected catchment, reuse demand and treatment configuration all influence the result.
No. A project may contain some untreated impervious surfaces and still achieve an overall rating of 100%.
For example, a development may include:
The assessment considers the combined performance of the development rather than requiring a separate treatment device for every small surface.
However, untreated areas must still be included in the assessment. They should not be omitted to make the result appear stronger.
The final rating should represent the complete proposed development, including both treated and untreated catchments.
No. A 100% STORM rating is not the same as retaining every litre of rainfall within the property.
Stormwater treatment measures may:
A rainwater tank, for example, may capture roof water until it reaches capacity. Once full, additional runoff may pass through an overflow connection.
A raingarden may filter runoff before excess water leaves through an underdrain or overflow.
The benchmark concerns treatment performance. It does not require the development to operate as a completely closed water system.
A 100% treatment rating is an important result, but it should not be interpreted as universal approval of the whole stormwater design.
The responsible authority may also need to consider:
A 100% rating should therefore be understood as evidence that the modelled water-quality benchmark has been achieved, not as automatic approval of every part of the project.
A council may question or decline to rely on a 100% result where the supporting inputs do not accurately reflect the proposed development.
Potential concerns may include:
The quality of the result depends on the quality and credibility of the assessment inputs.
A valid submission should show not only that the numerical benchmark has been reached, but that the treatment strategy can be incorporated into the proposed development.
Some assessment outputs may show treatment performance above the minimum benchmark.
This can occur where the development includes more treatment capacity than is needed to reach the reference objective.
For example, a project may have:
A result above 100% can provide additional design resilience, particularly where small changes may occur later.
However, a higher number is not automatically a better design if it depends on oversized, impractical or poorly coordinated treatment measures.
The objective should be a credible and maintainable strategy rather than the highest possible calculator result.
In some projects, a modest margin above the benchmark can be useful.
A design that reaches exactly 100% may fall below the target if later revisions:
A small performance margin can make the assessment less vulnerable to minor design changes.
This should not result in unnecessary treatment infrastructure. The margin should remain proportionate to the project and the likelihood of design development.
A rating below 100% generally means the current treatment strategy does not yet achieve the benchmark represented by the assessment.
It does not necessarily mean the project is fundamentally unsuitable.
The result may be improved by reviewing:
The most effective response depends on which catchments and treatment measures are limiting the result.
A project should not automatically respond by increasing the tank size. Other changes may be more practical or provide a stronger improvement.
Rainwater tanks can make a significant contribution where they capture runoff from a substantial roof area and serve regular non-potable water demands.
The main factors are:
Regular reuse creates capacity within the tank for future rainfall.
A large tank with little connected roof area or limited reuse may contribute less than expected. The tank, catchment and demand should be considered as one coordinated treatment system.
Read Can Rainwater Tanks Improve a STORM Rating? for a detailed explanation.
Where a tank alone does not achieve the target, additional treatment may be provided through other site measures.
A raingarden can treat runoff from roofs, driveways or paved areas through filtration and biological processes. Its contribution depends on its treatment area and connected catchment.
A genuine permeable pavement system can reduce the amount of untreated impervious area and may provide filtration, temporary storage and infiltration.
Suitable landscaped or engineered infiltration areas may help manage runoff where site conditions support their use.
Removing unnecessary paving or replacing sealed surfaces with permeable alternatives can reduce the runoff load that needs to be treated.
The strongest solution is often a coordinated treatment strategy rather than reliance on one oversized measure.
The original STORM Calculator has historically been used for relatively small Victorian developments.
For new suitable projects, current submissions may instead use BlueFactor or another method accepted by the responsible authority.
Older council correspondence and permit conditions may continue to refer to:
Although the software and output format may change, the underlying question remains similar: does the proposed treatment strategy achieve the required water-quality performance?
The accepted tool and submission format should be confirmed for the particular council and project.
A 100% STORM rating does not provide a complete drainage or flood assessment.
It does not ordinarily confirm:
These matters may need to be addressed through separate hydraulic, civil or flood-related documentation.
The STORM rating should not be presented as a substitute for those services.
The 100% benchmark gives project teams and councils a consistent way to test whether a proposed stormwater treatment response is likely to achieve the required water-quality outcome.
It can help:
The real value of the rating is not the number by itself. It is the clear connection between the result and a practical treatment strategy that can be implemented on site.
Confirm that the site area, roof areas, driveways, paving and landscaping match the current drawings.
Small untreated areas should remain in the model so the result represents the complete development.
Only assign runoff to a treatment measure where the physical drainage arrangement makes the connection achievable.
Toilet, laundry or irrigation uses included in the assessment should be shown in the appropriate plans or specifications.
Tank capacities, raingarden areas and permeable surfaces should remain consistent across the architectural, landscape and hydraulic documentation.
Confirm whether council also requires drainage plans, detention calculations, maintenance information or a broader Stormwater Management Plan.
Certified Energy can assess the proposed development and identify whether its stormwater treatment strategy reaches the required performance benchmark.
The assessment process may include:
Where the project requires MUSIC modelling, drainage design, detention calculations or other civil engineering documentation, that wider scope should be identified separately.
Explore the STORM Assessment Knowledge Hub or send through the current plans and council correspondence for an initial project review.
It generally means the proposed stormwater treatment strategy reaches the best-practice water-quality benchmark represented by the assessment tool.
No. The rating compares the project with defined pollutant-reduction objectives. It does not mean complete removal of suspended solids, phosphorus, nitrogen or litter.
Not necessarily. Some small areas may remain untreated where the combined project strategy still reaches the overall benchmark. All relevant areas should nevertheless be included in the assessment.
No. Treated runoff and overflows may still discharge to the drainage system. The rating relates to water-quality performance, not complete retention of all rainfall.
No. Council may also assess the accuracy of the report, consistency with the plans, practicality of the treatment measures and other planning and drainage requirements.
Some assessment outputs may show performance above the reference benchmark. The measures still need to be practical, maintainable and correctly documented.
A modest margin may be useful where future design changes are likely, but unnecessary or oversized treatment measures should not be added solely to maximise the score.
It generally means the current treatment strategy does not yet reach the reference benchmark and may need further refinement.
It may help, but tank performance also depends on connected roof area and regular reuse demand. A larger tank alone is not always the most effective solution.
No. Flooding, drainage capacity, detention and discharge may require separate civil, hydraulic or flood assessment.