Free stormwater tool · Every NZ territorial authority
Enter your site areas, get the required detention and retention volume, orifice size, and a suitable tank — then download a council-ready report. Built by a Chartered Professional Engineer.
Free to size and select. Each council’s own method · E1 & HIRDS rainfall · released as each is verified
Why it exists
Nearly every NZ council requires stormwater neutrality for new developments, but each has its own method, rainfall basis, and orifice rules. This tool encodes those council-by-council so you can size correctly the first time — without rebuilding a calculation sheet for every job.
Simple table lookups and full rational-method routing, matched to each council's rules — retention formulas, attenuation targets, and climate-change factors.
E1/AS1 tables for 141 locations plus HIRDS v4 multi-duration intensities, so the critical storm duration is found from actual data, not assumptions.
Get the volume and orifice, then pick a real tank from indexed suppliers and produce a report ready to attach to a consent application.
The process
Everything up to the report is free. You only pay when you download the final council-ready PDF.
The correct method, rainfall basis and rules load automatically, and only the routes that council actually offers are shown. Councils still being checked against their own source documents are greyed out.
Roof, impervious, and total site areas — or full pre/post development areas for the advanced rational method.
The tool returns required detention and retention volume, critical duration, and the orifice diameter to restrict discharge.
Select a suitable tank from indexed suppliers and download a council-ready PDF with the manufacturer specifications.
The deliverable
Coverage
Councils with a defined table or acceptable solution run as a guided free calculator. The rest use the advanced rational method, with council-specific requirements and submission notes built in.
Napier, Hastings, Timaru, Waitaki, Wellington Water councils, Kāpiti, Central Hawke's Bay, Christchurch, Auckland, Hamilton, Waikato, Waipā and more — sized directly from the council's own tables.
Whangārei, Tauranga, Rotorua, Dunedin, Queenstown-Lakes, New Plymouth, Palmerston North, Nelson, Tasman and the remaining authorities — full pre/post routing with each council's targets and climate factors.
Tank selection
Once you have the required volume, the tool suggests suitable tanks from an indexed product database — above-ground and underground — so your report references a specific, buildable solution.
Questions
Yes — sizing your tank, checking the orifice, and selecting a suitable product are all free. A charge applies only when you download the final council-ready PDF report. That payment step is being added; for now the full flow is open.
Councils are opened one at a time, as each method is checked against that council’s own source documents. Napier City Council offers three routes: the acceptable-solution table, the SW-S1 volume method, and a limiting-discharge site-specific design. Hastings District Council runs its own two-event On-Site Detention method. Configurations for the remaining territorial authorities are built but greyed out until verified. If you need a specific council, get in touch and we will prioritise it.
E1/AS1 tables for 141 NZ locations and HIRDS v4 multi-duration intensities. The advanced method routes across durations to find the critical storm rather than assuming one.
The report is structured for consent submission and includes the method, sizing, orifice design, and selected tank specifications. As with any sizing tool, the design should be reviewed and signed off by the responsible engineer for your project.
SAE Ltd (Subdivision and Engineering), a New Zealand civil engineering consultancy led by a Chartered Professional Engineer, specialising in stormwater and subdivision infrastructure.
Free to size, check the orifice, and choose a tank. No sign-up to start.