Published May 2026 · 9 min read

The Blue Line Through the House: What Overland Flow Paths Actually Mean for Auckland Property Buyers

A plain-English guide from the civil engineers at Know Your Risk NZ

You're looking at a property in Mt Albert, or Glen Eden, or Howick. You pull up Auckland Council's GeoMaps to check the natural hazards layer, and there it is. A thin blue line running straight through the dining room.

The agent hasn't mentioned it. The listing photos don't show it. Your heart sinks a little.

That blue line is an overland flow path (OFP), and it's one of the most misread items on any Auckland LIM report. Sometimes it matters a lot. Often, it matters less than the line makes it look. Here is what it actually is, why the council shows it going through buildings, and how to think about it as a buyer.

What an overland flow path is, and what it isn't

An overland flow path is the route surface water is predicted to take across the ground when the underground stormwater pipes are full. When the pipe network is overwhelmed in heavy rain, water doesn't stop falling. It just finds the lowest available path overland and keeps moving.

Auckland Council's Unitary Plan (Chapter J definitions) defines an overland flow path as a low point in terrain, excluding a permanent watercourse or intermittent river or stream, where surface runoff will flow during rainfall events, with an upstream contributing catchment exceeding 4,000m².

The 4,000m² catchment threshold is the regulatory trigger. Above that, the path is significant enough that the Auckland Unitary Plan starts caring about what you build on or near it.

Across the Auckland region, there are roughly 94,000 kilometres of mapped overland flow paths. That is not a typo. They are everywhere. In many suburbs they are closer to the rule than the exception. An OFP on your LIM is normal, not a red flag in itself.

It is also worth being clear what an OFP is not.

It is not a floodplain. A floodplain is the area inundated when a stream or river overtops its banks. An OFP is the path water takes overland when the stormwater pipes can't cope. The two often overlap in low-lying areas, but they are separate things on the council's maps.

It is not evidence the property has flooded. It is a model prediction, not a history.

It is not an automatic insurance problem. Most insurers cover OFP-related flooding under standard policies, though you should always check.

Why the line goes through buildings (and why that's misleading)

This is the part that confuses almost every buyer who sees an OFP on a council map for the first time.

Auckland Council's regional OFP map is built from a bare-earth model of the ground. It uses LiDAR data flown over Auckland in 2016 (with newer captures in some catchments), processed through a standard hydrological algorithm called D8 in ArcGIS. The LiDAR sensor records multiple laser returns as it flies, but the model only uses the last return, which represents the ground surface.

That means buildings, fences, and other above-ground structures are deliberately stripped out of the data before the flow paths are drawn. The Auckland Council open data layer for OFPs is explicit about this. The model is showing you where water would go if the house wasn't there.

So when you see a blue line running through a dining room, it is not saying “this room floods.” It is saying: “If you flattened that building to bare ground, water from upstream would naturally drain along this line.” What the water actually does when it hits the building is a different question, with several possible answers.

It flows around the building. Most common outcome. The structure acts as an obstacle and water diverts to either side, often along the boundary.

It flows under the building. If the house is on piles with an open subfloor, water can pass straight through underneath. The line on the map is showing the natural path, and underneath the floorboards is, hydrologically, still part of the natural path. A lot of older Auckland villas and bungalows sit this way. The water arrives, passes through the subfloor space, and leaves on the downstream side without ever touching a habitable room.

It backs up against the building. If the building sits perpendicular to the flow and has no opening underneath (slab-on-grade, blocked subfloor vents, solid garage walls), water can pond on the upstream side until it spills around. This is the scenario where you can get water entering through doorways, vents, or low brickwork.

It saturates the surrounding land. Even when the building itself is fine, the property around it may sit wet for days. More on this below.

The blue line is a planning tool, not a forensic prediction. It is telling the council and the buyer to pay attention to this corridor. It is not telling you the house has water in it.

Sheet flow, channel flow, and what the line is really depicting

This is where it pays to understand a bit of stormwater hydraulics.

The blue line on Auckland Council's map is what engineers call the thalweg. That is the lowest point on a cross-section of the flow path. Think of it like the centreline of a very shallow, very wide creek that only exists when it is raining hard.

What it doesn't show you is the width or depth of flow. And the council is upfront about this. The practice note (RC 3.2.18) and the GD13 Freeboard guideline both state that the mapped lines represent the centreline only, “estimated for shallow sheet flows”, and that they do not show the extent of the flow.

This matters because most overland flow in residential Auckland is sheet flow, not concentrated channel flow.

Sheet flow is water moving as a thin film across a broad area. Typical depths in residential OFPs are a few centimetres up to maybe 100mm in a 1% AEP storm (a storm with a 1 in 100 chance of happening in any given year). The water might be spreading over a corridor several metres wide. It is wet, it is annoying, it might soak into your garage if the floor level is wrong, but it doesn't have the destructive energy of fast-moving channel flow.

Concentrated or channel flow is what you get inside a defined depression. A cut drain, a steep gully, a kerb-and-channel down a sloped street, a creek bed. Here the water collects, picks up speed, and gets dangerous. Depths can be half a metre or more, velocities can knock a person off their feet, and debris loads get serious.

The takeaway: unless your blue line follows a clearly defined channel or gully, you are almost certainly looking at a sheet-flow corridor. That is a manageable situation in most cases. It is a “wet feet and a soggy garden” situation, not a “kayak through the living room” situation. There are exceptions, particularly in steep terrain and at the bottom of long catchments, which is exactly the kind of nuance a site-specific assessment is for.

The thing nobody talks about: your land getting wet

Most OFP discussions focus on whether water enters the building. That is the wrong question for a lot of properties.

If the blue line crosses your section, even if it misses the house completely, your land is going to get wet during major storms. That has real consequences that buyers rarely think about up front.

None of this is catastrophic. None of it shows up on a LIM in any useful way. But it is the kind of thing a buyer wants to know before settling, because the cost of dealing with it (re-grading, re-routing, French drains, raising floor levels) sits with the new owner, not the old one.

What the Auckland Unitary Plan actually says about building near an OFP

The regulatory framework matters because it affects what you can do with the property, particularly if you are planning to renovate, extend, or develop.

Under the Auckland Unitary Plan (and its Plan Change 120 amendments, notified in November 2025), the rules scale with the catchment size of the flow path.

For an OFP with an upstream catchment less than 4,000m², buildings and structures within it are generally a permitted activity. Lighter touch.

For an OFP with an upstream catchment greater than 4,000m² (the regulatory threshold), buildings and structures within the path become a restricted discretionary activity. That means resource consent may be required, and the council will look at how the flow path is maintained, how habitable rooms are positioned relative to the path, and how the building handles safe access during a flood event.

Diverting, piping, or reducing the capacity of any OFP is also a restricted discretionary activity. You can't legally just fill it in or put a fence across it without consent.

Practical implication for buyers: if the line crosses the property and you are thinking about adding a granny flat, building a deck, putting in a solid fence, or doing a major landscape change, factor in the possibility of needing council sign-off. The cost and delay isn't enormous in most cases, but it is real, and worth pricing into the project.

What to actually do as a buyer

If a property you are considering has an OFP across it, the order of operations is something like this.

Look at the topography yourself. Walk the section. Where does the land slope? Where would you expect water to go? Is the building elevated, on piles, on a slab, or cut into a bank? Are there obvious low points? Auckland Council's GeoMaps lets you turn on contour lines. Have a look.

Check the upstream catchment. Where is the water coming from? A 5,000m² catchment of suburban roofs and roads behaves very differently from a 50-hectare bush gully. The larger the catchment, the more water you are dealing with in a storm.

Order a LIM report. The OFP notation will be on it formally, along with any consent history or freeboard requirements the council has imposed in the past. A LIM costs around $375 and takes about 10 working days through Auckland Council.

Get a Know Your Risk report first if you are early in the process. Our reports are instant, cost $49, and tell you whether an OFP is present, what the upstream catchment size is, and how that flows through the PC120 framework, all before you commit to a LIM. We are not a substitute for a LIM at settlement, but we are a much faster way to triage properties when you are looking at several at once.

Talk to a stormwater engineer if the property is high-value, if you are planning development, or if the OFP looks like it concentrates into a channel rather than spreading as sheet flow. A site-specific hydraulic assessment can confirm flow depths, velocities, and the actual extent of the path. The council will accept this kind of report and update the GIS layer if your assessment shows the modelled line is wrong.

Check the building consent history. If the house was built after the OFP was mapped, the consent may already include conditions on floor levels, drainage, or flow path management. That is useful information.

Look at insurance. Most policies cover surface water flooding from OFPs, but exclusions vary. Ask your broker directly.

The bottom line

An overland flow path is the council telling you, “In a big enough storm, water travels through here.” It is a planning tool drawn on a bare-earth model that doesn't see your house. The line shows the centre of a corridor that is often wider, shallower, and less dramatic in practice than the map suggests.

For most Auckland properties with an OFP, the right response isn't panic. It is paying attention. Understand where the water comes from, where it goes, what the building is sitting on, and what the council's rules say about modifying the corridor. That gets you 80% of the way to an informed decision.

The remaining 20%, if you need it, is what a Know Your Risk report and a site walk will tell you in an afternoon, and what a stormwater engineer can confirm in a week.

The blue line is information. It is not a verdict.