Why Your Council Flood Maps Might Be Outdated - And What We're Doing About It
If you've ever pulled up your council's flood maps to check a site, you've probably assumed the data is current. For most of New Zealand, it isn't. The overland flow path layers shown on council GIS platforms are typically derived from LiDAR captures that are five to ten years old. In that time, the ground has changed - and so have the flow paths.
Today, SAE Ltd is launching a free interactive overland flow path map for Auckland built from the latest 2024 LINZ 1 m LiDAR. It's the most detailed publicly available overland flow path dataset for the Auckland region, and it's free for anyone to use.
The problem with older LiDAR
Auckland Council's GeoMaps overland flow path layer is based on a LiDAR baseline captured around 2016. That's roughly eight years of development that isn't reflected in the data - earthworks, new subdivisions, roads, retaining walls, re-graded sites, and filled gullies.
Overland flow paths are driven by terrain. When the terrain changes, the flow paths change. A retaining wall on a boundary can redirect a flow path onto a neighbouring property. A new road can concentrate sheet flow into a channel that didn't exist before. Fill placed across a natural swale can push water somewhere it's never been.
If the LiDAR doesn't capture those changes, the mapped flow paths are wrong. Not slightly wrong - potentially directing you to assess the wrong part of a site entirely.
This isn't unique to Auckland. Councils across New Zealand face the same issue. LiDAR is expensive to capture, and updating flood mapping layers isn't always a budget priority. The result is that planners, designers, and consent authorities are making decisions based on terrain data that no longer matches reality.
What we've built
We've processed the 2024 LINZ 1 m LiDAR DEM for the entire Auckland region using GRASS GIS and the r.stream.extract module. This uses an A* search algorithm to generate connected, source-to-sea overland flow paths - not just disconnected segments, but continuous drainage lines from ridgeline to coast.
The result: 1,169,441 individual flow paths covering the Auckland region.
We've published this as a vector tile layer using PMTiles, served through a MapLibre GL JS web map at subeng.nz/flood-map.html. You can zoom into any property in the Auckland region and see the overland flow paths crossing or adjacent to your site.
Key details:
- Source data: 2024 LINZ 1 m LiDAR DEM
- Processing: GRASS GIS r.stream.extract (A* algorithm)
- Coverage: Full Auckland region
- Flow paths: 1,169,441 connected source-to-sea lines
- Cost: Free to view - no login, no paywall
Who this is for
This map is a screening tool. It's designed for anyone who needs a quick read on whether overland flow paths affect a site:
- Planners and resource consent officers checking whether a site has overland flow path exposure before requesting further assessment
- Developers and landowners scoping a subdivision or building project and wanting to understand flood risk early
- Surveyors and architects checking site constraints before committing to a design layout
- Civil engineers doing initial desktop reviews for stormwater design
If you can see a flow path crossing your site on this map, it's worth investigating further. If you can't see one, that's a good sign - but it doesn't guarantee the site is free from flood risk. Sheet flow, localised ponding, and pipe capacity issues won't show up in an overland flow path analysis alone.
What this map doesn't do
To be clear: this is not a substitute for a professional flood assessment. The map shows where water is likely to flow based on terrain, but it doesn't tell you:
- How deep the flow will be in a design storm event
- What the velocity and flood extent will be
- Whether the flow path is classified as OLFP1 or OLFP2 under Auckland Council's guidelines
- What building platform levels or setbacks are needed
- Whether a resource consent or building consent application will require a flood hazard assessment
For those answers, you need a site-specific assessment from a qualified engineer. SAE Ltd provides consent-grade flood assessment reports starting at $3,500+GST, including overland flow path analysis, flood level estimation, and recommendations for building platforms and mitigation.
What's coming next
Auckland is the first region, but not the last. We're currently processing LiDAR data for Hastings and Napier, with plans to roll out coverage across the rest of New Zealand as LINZ makes updated LiDAR datasets available.
Our goal is to build a nationwide overland flow path map that stays current as new LiDAR is captured. Every time LINZ publishes a new DEM, we'll process it and update the map. No more relying on council layers that are half a decade behind.
We're also working on integrating this data with our flood modelling pipeline - combining overland flow paths with HEC-RAS 2D hydraulic modelling to produce site-specific flood hazard reports at scale. That means faster turnaround and lower cost for developers who need consent-grade flood assessments.
Try it now
The Auckland flood map is live at subeng.nz/flood-map.html. Zoom into any property in the Auckland region and see the overland flow paths for yourself. No login required.
If you're working on a project and need to understand what these flow paths mean for your site, get in touch. We provide detailed flood assessment reports for resource consent, building consent, and subdivision applications across New Zealand.
You might also find our free NZ detention tank sizing calculator useful if you're sizing stormwater detention for a development.
Need a site-specific flood assessment?
SAE Ltd provides detailed overland flow path analysis and flood hazard reports for resource consent, building consent, and subdivision applications across New Zealand.