Auckland's stormwater standard is being updated for the first time since 1999. Here's what TP108 (2026) means for your site.

When an engineer models how stormwater leaves your site, the first number they reach for is a rainfall depth - and in Auckland, that number has come from the same document since 1999. Auckland Council is now updating it. If you have a site in feasibility, in early design, or heading toward consent in the next twelve months, put this in front of your engineer before the final version lands, not after.

What TP108 is

TP108 sets out how stormwater runoff is modelled across the Auckland region. It is the document behind the numbers that matter on a development site: flood levels, detention requirements, minimum floor levels. Almost every stormwater calculation a project relies on starts from the rainfall inputs it provides.

Those inputs date from 1999. The science has moved a long way in more than twenty-five years - including several storms that rewrote what Auckland thought an extreme event looked like. The document has not.

What is changing

Auckland Council has published a proposed update - TP108 (2026) - and opened it for industry feedback. It is a partial update, and the document is precise about what that means.

Appendix A (Design Rainfall Maps and Data) has been fully replaced. It supersedes all previous rainfall mapping and the design rainfall depths that went with it, and where anything elsewhere in the document conflicts, Appendix A takes precedence. The rest of the document stands: the hydrological methodology and analysis from 1999 remain valid guidance.

So the method does not change. The numbers going into it do - practitioners are directed to "apply the updated rainfall inputs in conjunction with the existing methodology". On paper that is one appendix. In practice it is the number the whole model is built on.

The old contour maps and their multiple isohyets are gone. In their place are two rainfall zones. Zone A covers most of the region, including nearly all of urban Auckland. Zone B is the higher-rainfall country - around the Hūnua and Waitākere Ranges, the Mahurangi Hills, and high-elevation Great Barrier Island.

The proposed depths

The comparisons in the last column are Auckland Council's own. These are consultation figures and could still move before publication.

Where it bites: feasibility

The stage where this matters most is the stage where it is easiest to miss - and the pattern in that table decides how much it matters to you.

Read down the right-hand column. Everyday events barely move. The drift grows as events get rarer. Against the worked example inside TP108 itself, the 2-year storm goes from 75mm to 80mm and the 10-year from 130mm to 140mm - but the 100-year goes from 200mm to 250mm, a 25 per cent increase in the figure underneath every flood calculation on that catchment.

So the question on your site is not "what does this cost". It is which event governs the design:

• Routine detention sizing leans on the frequent end, where almost nothing has changed.
• Flood levels and minimum floor levels lean on the rare end, where the movement is. Raise those and you are into earthworks, retaining, driveway grades and how the units relate to each other.
• Flood extents, depths and velocities move with them - which is hazard assessment, overland flow paths, and how much of the site is comfortably developable.

None of these are small adjustments once a project is priced, designed and lodged. They are the kind of change that moves a feasibility from workable to marginal, and the later they surface, the more they cost to absorb.

It can also go the other way, and Council's own comparisons say so: on some sites and some events, the new depths are lower than 1999. The risk is not that every site gets harder. It is not knowing which kind of site you are holding.

Two things that are easy to miss

Climate change adjustments do not come from TP108. They come from the Auckland Council Stormwater Code of Practice and apply on top of these depths. Council has confirmed the 3.8°C allowance still stands for secondary system design and floodplain mapping, and that this is not double counting - the updated statistics only account for warming already in the record.

Council's published flood maps have not caught up. The flood layers on GeoMaps and Flood Viewer still run on TP108 (1999) rainfall. If you are doing due diligence on a site this month, the flood extent you are looking at is built on the old data. Those layers will pick up the new depths as they are revised in future.

The dates that matter

• 7 September 2026 - the feedback period closes. A narrow window for practitioners to influence the final document.
• Q4 2026 - the expected formal adoption date.
• Before then - Council will share a detailed implementation plan covering resource consents, engineering approvals and building consents, including how in-progress applications are handled.

Council says the industry is welcome to adopt the updated approach now, and recommends practitioners prepare to transition. If your project consents in late 2026 or through 2027, work on the assumption that the updated standard will apply. Waiting for the implementation plan to find out is leaving it late - by then the design is locked and the numbers are already in the budget.

This is a regional change, not a localised one. Every Auckland development site is potentially in scope.

What to do before 7 September

1. Get your zone and your governing event. Which zone is the site in, and which ARI drives the design? Together those two answers tell you whether this is a non-event or a real change. A GIS layer of the new zones is available on request from tp108@aucklandcouncil.govt.nz.
2. Re-check anything resting on a 100-year level, and confirm the climate change uplift is being drawn from the Stormwater Code of Practice, as the update requires.
3. Check your timeline. Anything lodging after the final version publishes should be tested against the new inputs, not the 1999 ones.
4. Submit feedback if it affects you. The people who use this document every week are best placed to comment on it. Council's form is at forms.office.com/r/rf9eSupGbk, and the window closes 7 September.

Where we stand

At Creston Property we manage the full development process - feasibility, consenting and build management. Part of that is tracking changes like this one and working out what they mean for a site before money is committed.

We will not pretend everything can be seen coming. Construction is a hard industry, and some costs stay hidden until the ground is open. What early work does is shorten the list of surprises. A rainfall depth published in a public table is a knowable thing - and knowing it at feasibility is a far cheaper problem than meeting it at consent.

That is the distinction that matters here. Some risks can be found early if someone goes looking. Some cannot. TP108 (2026) is firmly in the first group.

If you are working through feasibility on an Auckland site and want a development partner who goes looking, start with a conversation. Get in touch or visit www.crestonproperty.co.nz.