How to read a NSW flood study for one property: a walkthrough

A published NSW flood study gives you more than a yes-or-no overlay flag: it carries a modelled flood depth, a flow velocity, a depth times velocity hazard rating and a flood level in metres AHD for each design event at your location. Reading it in the right order turns those numbers into the two things a buyer or builder cares about: how high the water gets against the house, and what that means for a floor level and an insurance conversation.

This walkthrough works for the flood studies and floodplain risk management studies that NSW councils and the state hold for catchments across Sydney, Newcastle, Wollongong and regional NSW. Not every address sits in a modelled catchment, so the first task is confirming a model reaches the lot at all.

Step 1: Confirm a model actually covers the address

Start by checking whether a published study reaches the property, because many NSW catchments are mapped and many are not. Councils prepare flood studies catchment by catchment under the NSW Floodplain Development Manual, so an older or rural catchment may have no study, while the neighbouring suburb has a detailed one.

The gotcha: a property can sit inside a council area that has flood studies without being inside a modelled catchment. If the lot falls in a gap between studies, no depth or level exists for it, and absence of a number is not the same as no flood risk.

Step 2: Read the section 10.7 certificate first

Pull the section 10.7 planning certificate before you open any map. The certificate tells you whether the land is flood affected under the Local Environmental Plan and whether development controls apply, which frames everything the model then quantifies. Our guide to how to read a section 10.7 certificate for ground hazards NSW walks through the clauses.

The gotcha: the 10.7 certificate states that controls apply, not the modelled depth or level. It is the planning flag. The flood study behind it holds the measured numbers. Treat them as two documents doing two jobs. The difference between the flag and the model is covered in flood certificate vs flood model report.

Step 3: Identify the design events in the study

Find which events the study ran. NSW flood studies are usually reported by Annual Exceedance Probability (AEP), the chance of an event being equalled or exceeded in any one year. The 1% AEP event, the old 1 in 100 Average Recurrence Interval (ARI), is the standard planning event. Many studies also report the 5% AEP, 1% AEP, 0.5% AEP and the Probable Maximum Flood (PMF).

Term	Meaning	Older name
5% AEP	5% chance in any year	1 in 20 ARI
1% AEP	1% chance in any year	1 in 100 ARI
0.5% AEP	0.5% chance in any year	1 in 200 ARI
PMF	Largest flood considered credible	-

The gotcha: the 1% AEP is not the worst case. The PMF can sit well above it, and NSW councils increasingly use the PMF to set evacuation and emergency planning. Read both.

Step 4: Read the modelled flood level in metres AHD

Read the flood level for the 1% AEP event at the lot. It is reported in metres Australian Height Datum (AHD), a fixed national reference, not metres above the ground. A level of 4.2 m AHD means nothing until you compare it with the ground surface level at the house.

The gotcha: a level in AHD is only useful next to the ground level in the same datum. A low AHD flood level on high ground is harmless; a modest one on a low lot is not. Get the surface level from a contour survey or elevation data for the lot.

Step 5: Work out the depth over the floor

Subtract the ground level from the flood level to get depth, then compare it with the proposed or existing floor level. NSW councils set a Flood Planning Level, usually the 1% AEP level plus a freeboard allowance of around 500 mm, as the minimum habitable floor level. Your floor should sit above it.

The gotcha: freeboard is an allowance for model uncertainty and wave action, not spare height to spend. Designing the floor exactly to the Flood Planning Level leaves no margin.

Step 6: Read velocity and the depth times velocity hazard

Check the velocity in metres per second and the depth times velocity (D x V) hazard classification. NSW uses the hazard categories from the Australian Disaster Resilience guideline, H1 through H6, which combine depth and velocity to describe the danger to people, vehicles and buildings. A shallow but fast flow can be more dangerous than deep still water.

The gotcha: a low depth does not mean a low hazard. In a steep Wollongong or Blue Mountains catchment, velocity drives the rating, and a floodway where water moves fast carries far tighter controls than a flood storage area.

Step 7: Check for a future-climate run

Look for whether the study includes a climate change scenario. Many recent NSW flood studies model the 1% AEP event under an increased rainfall intensity, often shown as a separate layer with a higher level than the present-day run.

The gotcha: an older study may carry only present-day results. If your floor is set to a present-day 1% AEP level, a future-climate level several hundred millimetres higher changes the picture over the life of the building.

Step 8: Take the numbers into design and insurance talks

Use the flood level, depth, velocity and hazard together when you talk to a designer, a certifier or an insurer. An insurer prices on modelled depth over the floor, so a documented floor level comfortably above the Flood Planning Level is a stronger position than a vague overlay flag.

The gotcha: a desktop reading of a published model scopes the conversation and sets expectations. It is not a survey and not engineering advice for the lot. A surveyed floor level and a designer confirm what the numbers mean for the specific build.

To see whether a published model reaches a specific address, check whether a flood model covers the address, or if you want zoning and the planning flags alongside it, get a Property Report instead.

LayeredGeo's Flood Model Report collects every published flood model that reaches a property in the areas we hold across Queensland, New South Wales, Victoria, the ACT and Tasmania, with modelled depth, velocity and flood level for each event.

About this article. Published by LayeredGeo. It is general information about how property and ground conditions are assessed in Australia, not engineering, planning, legal or financial advice, and it is not specific to any property. Check anything that matters against the source dataset or a suitably qualified professional before you rely on it. If you spot something wrong, tell us at hello@layeredgeo.com.au and we will fix it.

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