Overland Flow Paths: Check a Property Before You Buy
Where surface water runs across a New Zealand section in heavy rain, why the council map might not show it, and what to ask before you commit.
An overland flow path is the route rain takes across the ground when it falls faster than the soil and the pipes can take it. Most of the time it is invisible: a slight dip along a back fence, a shallow line down a lawn, a driveway that happens to sit in the low point of the street. In a heavy storm it carries water off every roof, road and paddock above it, and whatever sits in the way gets wet.
That makes it one of the most useful things to know about a property before you buy it, and one of the easiest to miss. A flow path across the only flat building platform can mean a consent, a raised floor, a redesign or a different section. None of that shows up in a listing photo taken on a dry day.
Why the council map might not show it
Councils map flood hazard for a whole district, and the maps reflect that. Some model only rivers and large floodplains. Some show smaller overland flow paths as an indicative layer that the council itself says needs checking on site. And some were built from LiDAR flown years before the current survey, before the subdivision, the new road or the earthworks next door changed where the water goes.
None of that makes the council map wrong. It is the layer consent decisions are made on, and a LIM will tell you what the council knows. But a path draining a hectare of suburban hillside can sit below what a district-wide model was built to show, and the only way to see it is to look at the ground itself.
Auckland’s 4,000 m² rule
Auckland is the clearest case because the Unitary Plan puts a number on it. An overland flow path is defined as a low point in the terrain, other than a stream, where surface runoff flows from an upstream catchment of more than 4,000 m², or 0.4 ha. Works that divert, pipe or reduce the capacity of one fall under the natural hazards and flooding rules in Chapter E36 and generally need resource consent. The flow path layer on Auckland Council’s GeoMaps is indicative; whether a particular low point meets the definition comes down to the size of the catchment above it.
That is why the hectare figure on each path matters. Other councils write their rules differently, but the question is always the same: how much land drains through this point, and what happens if I put something in the way?
Map the flow paths on a property
Select a parcel and run Flow Paths. Tap any line to see how much land drains through it. Free to use, no account.
Open Flow Paths →Check a property in six steps
Select the property
Search the address and click the parcel, so the analysis is clipped to the legal title. The land above the boundary is still read, so water arriving from the neighbours is counted at its true size.
Property →Run Flow Paths
Each line is a path surface water would take in heavy rain, drawn thicker and paler the more land drains through it. Look first at the house site, the driveway and anywhere you might put a garage, a septic field or a retaining wall.
Flow Paths →Tap the lines that cross the section
Tapping a line shows how many hectares drain through it. A path from a few hundred square metres of lawn is a drainage detail. One carrying runoff from several hectares of the hill above is a design constraint, and in Auckland anything over 0.4 ha meets the Unitary Plan definition.
Flow Paths →Switch to Ponding
Ponding shows where water would stand once the hollows fill, with depth, area and volume. A flat back lawn that turns out to be a 40 cm-deep basin is worth knowing about in October rather than discovering in July.
Flow Paths →Trace where the water comes from
Click the point where the main path enters the section. Catchment maps all the land upstream of it, which tells you whether the water is coming off your own slope, a neighbour’s block or a road.
Catchment →Compare with the council
Open the council’s hazard viewer for the same address and read the LIM. The Flow Paths panel names the LiDAR survey it was traced from and the year it was flown. If the council layer pre-dates it and the two disagree, that is a specific question to put to the council or an engineer.
LiDAR coverage and survey dates →Reading the hectare figure
Every path on the map is classed by the land draining through it, in fixed hectare bands, so a line means the same thing on a 600 m² section as it does across a 40 ha block. The smallest path drawn on a property-sized area drains 0.1 ha.
| Line | Land draining through | What it usually means on a property |
|---|---|---|
| Minor flow path | under 0.4 ha | Runoff off the section and its neighbours. A drainage detail for a builder or landscaper. |
| Overland flow path | 0.4–10 ha | Meets Auckland’s definition. Keep buildings out of it, or expect consent and design work. |
| Watercourse | 10–100 ha | A gully or drain carrying a sizeable hillside. A real constraint on where you build. |
| Stream or river | over 100 ha | A stream or river. Check the council’s flood maps; this map does not model river floods. |
An example: a week before going unconditional
Illustrative, not a real property. A buyer has a 900 m² section under contract on a gentle slope below a cul-de-sac. The council viewer shows no flood overlay on the site. Running Flow Paths on the parcel shows a line entering from the road and running diagonally across the flat part of the section, the part the buyer planned to build on. Tapping it reads 0.7 ha: the cul-de-sac, the berms and the backs of a dozen houses above. Catchment confirms the water is coming off the road, not the buyer’s own slope.
That does not mean the section floods. The road may well have sumps and pipes that carry most of it away, which the terrain cannot see. What it gives the buyer is a specific, answerable question for the council and an engineer, asked while the contract is still conditional, instead of a surprise at the consent stage.
Put it in the file
The property terrain report runs flow paths alongside contours, slope, aspect and winter sun over the title boundary, names the LiDAR survey, and ends with the checklist of council records the terrain cannot cover.
What this cannot tell you
- How deep or fast the water would be. The map shows where the ground sends water, not what a 1-in-100-year storm would do. That needs rainfall, pipes and a hydraulic model.
- Whether the pipes cope. Stormwater pipes, sumps, culverts and kerbs are not in the terrain, so the map shows where water goes when they are full or blocked.
- What the buildings do. Bare-earth LiDAR has houses removed, so paths are drawn through existing buildings.
- River and coastal flooding. Large rivers, storm surge and sea-level rise are modelled by regional councils; use their hazard maps.
- The legal position. Only the council can tell you how its rules apply to a property. Get the LIM.
Related guides
- Terrain checks before buying rural land — seven checks for a lifestyle block.
- Slope maps for building on sloping land — where the buildable ground is.
- LiDAR coverage in New Zealand — which survey covers an address, and when it was flown.
Frequently asked questions
What is an overland flow path?
The route surface water takes across the ground when rain falls faster than the soil and the stormwater pipes can take it. It usually follows a shallow low line: a swale between houses, the back of a row of sections, a driveway, a gully. Auckland’s Unitary Plan defines one as a low point in the terrain carrying runoff from more than 4,000 m² of land upstream, not counting streams. Other councils use similar ideas under their own rules.
How do I find out if a property has an overland flow path?
Three sources, used together. Your council’s GIS viewer, which in many districts shows flood-prone areas and in some shows overland flow paths. The LIM, which records what the council knows about the property. And the terrain itself: an overland flow path map traced from the LINZ 1 m LiDAR shows where water would run across the section and how much land drains through each path, including paths too small for a district-wide flood model.
Can I build over an overland flow path?
Often not without consent, and sometimes not at all. In Auckland, works that divert, pipe or reduce the capacity of an overland flow path are covered by the natural hazards and flooding rules in Chapter E36 of the Unitary Plan and generally need resource consent. Elsewhere it depends on the district plan and the Building Act. The practical answer for a buyer is to find out before going unconditional, because a path across the only flat building platform changes what the section is worth.
Why does the council flood map not show the flow path I can see?
Council flood maps are drawn for a whole district. Some only model rivers and large floodplains; some smaller-scale overland flow layers are indicative rather than surveyed; and some were built before the latest LiDAR was flown. A path draining a hectare of suburban land may simply be below what the council layer was built to show. Where the terrain and the council disagree, ask the council. Its layer is the one used for consent decisions.
Is an overland flow path map the same as a flood map?
No. An overland flow path map shows where the ground sends water and how much land drains through each path. A flood hazard map models a storm of a given size and shows how deep and fast water would run. The first tells you where to look; the second, usually produced by the council or by an engineer for a specific site, tells you how bad it would be.
What does a site-specific flood assessment add?
Rainfall for design storms (typically 10%, 2% and 1% annual exceedance probability), pipes and culverts, and a hydraulic model that gives water depth, speed and level at the property. Those levels are what councils use to set minimum floor levels. It is engineering work, signed off by a chartered professional engineer, and it is worth commissioning when a flow path crosses where you want to build.