Overview
FloodMap.nz publishes a consistent nationwide overland flow path dataset derived from the LINZ 1m LiDAR composite. Flow paths are classified into five tiers by upstream catchment area, alongside potential ponding areas. Site-specific flood assessments — adding HEC-RAS 2D hydraulic modelling, depth and velocity outputs — are scoped per site as bespoke engineering through SAE Ltd; the methodology for those is documented below for reference.
The dataset is produced from the DEM up. No single council's model is embedded in the output — every tile is reprocessed when new LiDAR is released.
Data sources
Used in the free map layer:
| Digital Elevation Model | LINZ New Zealand LiDAR 1m DEM composite (layer 121859), 2018–2025 surveys, the latest available for each region. CC BY 4.0. |
| Areas with no LiDAR | LINZ 8 m national DEM (2012), used only for an indicative layer of larger streams inside a dashed amber boundary. CC BY 4.0. |
| Flow path grid | A native 1 m grid in 20 council areas and a 2 to 4 m grid elsewhere, all derived from the 1 m LINZ source. Clicking a line on the map shows the grid it came from. |
| Basemaps | LINZ Basemaps (aerial, CC BY 4.0); OpenStreetMap contributors (streets, ODbL) |
| Address search | LINZ NZ Addresses (CC BY 4.0) and OpenStreetMap Nominatim |
Used only in commissioned site-specific assessments, under each provider's own licence terms:
| Land cover | LCDB v5 (Manaaki Whenua – Landcare Research), mapped to Manning's n |
| Soils | Fundamental Soils Layer + S-Map where published (Manaaki Whenua – Landcare Research), mapped to hydrologic soil group |
| Rainfall | NIWA HIRDS v4; TP108 for Auckland |
Five-tier catchment classification
Flow paths are classified by upstream contributing catchment area. Tier thresholds are set by upstream catchment area, using breakpoints commonly seen in stormwater practice, so the layer is straightforward to read alongside other datasets.
| Tier | Catchment (ha) | Typical use | Visible zoom |
|---|---|---|---|
| T1 | >5,000 | Rivers: regional context | z7+ |
| T2 | 500–5,000 | Rivers and major streams | z10+ |
| T3 | 25–500 | Streams | z13+ |
| T4 | 2–25 | Minor watercourses, subdivision layout | z14+ |
| T5 | <2 | Small overland flow paths (AUP E36 trigger = 4,000 m²) | z15+ |
All five tiers are published nationwide. The smallest Tier 5 paths go down to about 0.1 ha in the 20 native 1 m council areas and about 0.2 ha on the 2 to 4 m grid. Upstream areas on the largest rivers (T1) can read low where a catchment crosses a survey edge or a LiDAR gap, so treat them as indicative; for authoritative large-river hazard, use council flood maps.
Potential ponding areas are low points in the bare-earth terrain where water could pond before overflowing. Yellow dashed connector lines link where flow enters a ponding area to where it leaves; they are indicative links, not modelled flow paths.
Processing approach
Flow paths are derived from the LINZ 1 m LiDAR DEM using established open-source geospatial methods: the terrain is conditioned for depressions, flow accumulation is computed across the grid, and flow paths are extracted and classified by the upstream catchment area draining to each segment. The thresholds that separate the tiers are published in the table above.
The specific processing chain — the tools, parameters, block strategy and the order they are applied in — is proprietary to SAE Ltd and is not published. This is the same position taken in datasets supplied to councils. What is published is everything needed to judge whether the layer is fit for a given purpose: the source data, the classification thresholds, the standards it aligns with, and the limitations below.
No council's model is embedded in the output. Regions are reprocessed when LINZ publishes a new survey block covering them.
Site-specific assessments
Site-specific flood assessments are commissioned through SAE Ltd as bespoke engineering and are separate from this free layer. They add HEC-RAS 2D hydraulic modelling on top of the flow path network, producing maximum depth, velocity, depth × velocity and hazard outputs, and where required a before/after earthworks comparison and AEP depth at the property boundary. Rainfall follows TP108 or NIWA HIRDS v4 as appropriate to the region. See site-specific flood assessment for what is covered.
Standards alignment
The free layer's tier thresholds reference AUP E36 and NZS 4404. The other standards below apply to commissioned site-specific assessments.
| TP108 (Auckland) | Rainfall design storm, runoff coefficients |
| SWCoP v4 (Auckland Council Stormwater Code of Practice) | Manning's n tables, impervious surface definitions, OFP requirements |
| NZS4404:2010 | Land development and subdivision engineering |
| AUP E36 | Overland flow path regulation — 4,000 m² catchment trigger |
| HEC-RAS 2D User Guide (USACE) | Hydraulic modelling conventions |
| NRCS TR-55 | Curve number method (site-specific assessments) |
Where a regional council publishes its own stormwater code (e.g. Christchurch Waterways Wetlands and Drainage Guide), site-specific assessments for that region default to the local standard.
Limitations
What the OFP layer does not represent:
- Underground stormwater pipe networks, culverts, and constructed drainage are not modelled — flow paths are surface-terrain only
- The layer is a modelled terrain estimate, not a field-verified watercourse or an authoritative flood record
- Minor artefacts may occur at LiDAR tile boundaries, especially in flat or heavily developed areas
- Buildings and vegetation are represented by the LiDAR surface, which can produce spurious flow paths around large structures
- Upstream areas on the largest rivers (T1) can read low where a catchment crosses a survey edge or LiDAR gap; for authoritative large-river hazard, use council flood maps
- Flat ground can produce straight, parallel line artefacts, particularly across lakes, wide rivers and large flat areas
- Connector lines across ponding areas are indicative links, not modelled flow paths
- Coverage follows the LINZ LiDAR programme. Where LINZ has not published LiDAR, an indicative layer from the 8 m national DEM (2012) is shown inside a dashed amber boundary: positions are approximate and small streams can be missing
The free map layer is a general reference tool. Decisions on resource consent, building consent, property purchase, insurance, or infrastructure design should be based on a site-specific flood assessment, a LIM from your council, or advice from a qualified engineer.
Versioning & updates
Each published dataset carries a version tag showing the LiDAR survey vintage and pipeline version. The update cycle is:
- As published: affected regions are reprocessed when LINZ publishes a new survey block covering them
- Periodic: full nationwide rebuild to capture DEM composite updates and any methodology changes
- Major revisions: versioned with changelog announced via the contact channel; council partners are notified directly in advance
Current published version: v2.1 (October 2026): LINZ L121859 composite, five tiers T1 to T5 published, native 1 m grid in 20 council areas, potential ponding areas with connector lines, and an 8 m indicative layer where no LiDAR is published.