Overview
FloodMap.nz publishes a consistent nationwide overland flow path dataset derived from the LINZ 1m LiDAR composite. Flow paths are classified into six tiers by upstream catchment area. 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
| Digital Elevation Model | LINZ New Zealand LiDAR 1m DEM composite (layer 121859) — 2018–2025 surveys, the latest available for each region |
| Flow path grid | Flow routing is run on a downsampled grid, not at 1 m: 4 m nationally and 2 m in the published town centres, with a 2 m national rollout in progress. Every grid is an honest downsample of the 1 m LINZ source — there is no interpolation at any step. The grid applied to each region is shown in the map. |
| Land cover | LCDB v5 (Landcare Research, 2018) — 33 classes mapped to Manning's n |
| Soils | Fundamental Soils Layer (FSL) + S-Map where published; mapped to NRCS Hydrologic Soil Group (A/B/C/D) |
| Rainfall | NIWA HIRDS v4 (High Intensity Rainfall Design System); TP108 regionalised for Auckland |
| Basemaps | LINZ Basemaps (aerial); OpenStreetMap (streets) |
| Addresses | LINZ NZ Street Address (for geocoding + property reports) |
All data sources are publicly available under CC BY 4.0 or equivalent open licences.
6-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 | >500 | Regional rivers — use NIWA REC2 for authoritative | z7+ (Phase 2) |
| T2 | 100–500 | Major streams, district plan hazard zones | z7+ (Phase 2) |
| T3 | 25–500 | Streams (current published tier) | z7+ |
| T4 | 2–25 | Minor watercourses, subdivision design | z11.5+ |
| T5 | 0.4–2 | Fine OFPs (AUP E36 trigger = 4,000 m²) | z13+ |
| T6 | <0.4 | Property-scale OFPs (on-site drainage) | Phase 2 |
Published today: T3, T4, T5 nationwide. T1/T2 are reserved for authoritative sources (NIWA REC2) because 1m DEM block-boundary accumulation errors make large-catchment upstream areas unreliable. T6 is under development.
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
| 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
- T1 / T2 (large catchment) tiers are not yet published — for authoritative large-river hazard, use NIWA REC2 or council maps
- Coverage follows the LINZ LiDAR programme — areas LINZ has not yet flown are absent from the layer, and can be seen on the DEM coverage layer in the map
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: v2.0 (April 2026) — LINZ L121859 composite, 6-tier classification, T3/T4/T5 publishing.