How to check flood risk before buying a property

Riverine flooding is water leaving a river, creek or floodplain. Overland or stormwater flooding is rainfall that exceeds drains and runs through streets and lots. Coastal flooding is tide, storm surge or inundation on the coast. A lot can sit in more than one. A river overlay can miss a local overland path. A coastal code is not a riverine code.

Victoria’s Special Building Overlay is the state’s planning tool for overland flow from the urban drainage system. Its Land Subject to Inundation Overlay and Floodway Overlay are riverine (and some coastal) tools. South Australia’s flood overlays cover both riverine and surface-water flooding at a 1% AEP event. Tasmania splits riverine flooding (Flood-Prone Areas Hazard Code) from coastal inundation (Coastal Inundation Hazard Code). If you only search one family, you can miss the water that actually reaches the house.

Annual exceedance probability is the chance of a flood of that size being equalled or exceeded in any one year. A 1% AEP flood has a 1% chance in a given year. It is not a promise that the next one is 99 years away. The NSW Flood Risk Management Manual and Queensland Reconstruction Authority both use this language.

The probable maximum flood (PMF) is the largest flood that could conceivably occur at a location. In NSW it defines flood-prone land (the floodplain). The flood planning area is usually the land below the flood planning level, where everyday development controls apply. The flood planning level is typically a defined flood event (often 1% AEP) plus freeboard. NSW planning circular PS 21-006 says a 1% AEP event plus freeboard should generally be the minimum for residential flood planning levels, with other choices needing justification. Special flood considerations can apply between the flood planning area and the PMF for sensitive or hazardous uses.

Depth and velocity together describe hazard. Shallow slow water and deep fast water are different facts. Floor level is where the dwelling sits relative to the planning level. Egress is whether people can leave as water rises. A planning overlay rarely states all five. A council flood certificate or a hydraulic report is where those numbers live, when they exist.

Planning is a state and territory matter. The flood map you want is the one the local scheme or Code actually uses, plus any council study the scheme points to.

Climb only as far as the decision needs. Each rung costs more and answers a tighter question.

1. Screen. Official overlay or Code layer intersects the parcel. Districts, where loaded, reports intersectionRatio and materiality.
2. Planning text. The clause, schedule or Code overlay that the colour cites. This is what a new dwelling or subdivision would be assessed against.
3. Council or floodplain-manager report. Flood certificate, flood level advice or property flood report giving modelled levels for the address, when that product exists.
4. Disclosure document. NSW s10.7, Victorian s32, Tasmania s337, SA Form 1, or the searches that jurisdiction actually uses.
5. Insurance quote and lender position for this address, before the offer is unconditional.
6. Hydraulic or flood-engineer advice for a specific proposal, a high-hazard site, or when the first five rungs disagree.

Labelled illustration using Districts fields. A property page loads a flood overlay with materiality “affects” and intersectionRatio 0.18. Eighteen per cent of the official parcel sits inside the published polygon. The dwelling footprint is not in that field. You still do not know depth, velocity, floor level or egress.

A 3% rear-boundary clip and a 70% cover that includes the house are different facts. Some clauses apply to the whole lot once any part is mapped. Victoria can generate a definition plan that measures a partial overlay or bushfire-prone area against the title boundary. Use the share to decide which rung to climb next, not as a risk score.

Insurers use their own models and claims history. Lenders use their own policies. Emergency management uses evacuation and isolation, which can reach lots outside the flood planning area, up toward the PMF. A mapped overlay is a reason to ask those people. It is not their answer.

Absence of mapping is not an all-clear. SA’s Evidence Required overlay says so in the name. NSW council studies can sit outside the state LEP map. Tasmania’s statewide riverine layer is still being built. Mapping also lags new fill, new drains and new climate assumptions.

• Reading “1 in 100” as a calendar promise.
• Using Queensland DAMS as the council flood certificate.
• Treating SA Evidence Required as no flood, or a small Districts share as no issue.
• Ignoring overland flow because the river overlay is clear.
• Skipping the insurance quote until after exchange.

It does not establish floor level, depth, velocity, isolation, historic flooding, premium or a purchase decision. Districts’ flood share is a geometry calculation on official layers, labelled as such. Every flood result on a property page says it is not a flood certificate.

Lot-level flood is one of the ten checks. Where the layer is loaded, Districts intersects it with the official parcel, merges split tiles, and flags boundary-only touches. Where the layer is not retrieved, the check is unknown. See how to read Districts flood and planning data and the planning and flood methodology.

1. Official map in the table, dated in your notes.
2. Council or floodplain-manager product for this address.
3. Insurance quote and lender question for this address.
4. Conveyancer on the disclosure document used in that state.
5. A flood engineer if you will build, subdivide, or the first rungs disagree.