Flood Levels Explained: 1% AEP, PMF & Freeboard for Your NSW DA

If you have ever read a flood report, a planning certificate or a council DA condition and hit a wall of acronyms – 1% AEP, PMF, FPL, freeboard – you are not alone. These terms decide real things: how high your floor has to be, whether you can fill your block, how your driveway is graded, and sometimes whether a project stacks up at all.

This guide unpacks the flood terminology used in NSW in plain language, so that when your engineer or council assessment officer starts talking about flood planning levels, you know exactly what is being decided and why.

AEP: how flood probability is actually measured

AEP stands for Annual Exceedance Probability. It describes the chance that a flood of a given size will be equalled or exceeded in any single year. The benchmark event in most NSW development controls is the 1% AEP flood – a flood with a 1-in-100 chance of being equalled or exceeded in any given year.

You may know this event by its older names: the “1-in-100-year flood” or the “100-year ARI” (average recurrence interval). The industry has deliberately moved away from that language, because it misleads people in a dangerous way.

Why “1-in-100-year flood” is a misleading name

“1-in-100-year” sounds like a flood that happens once a century – so if one occurred recently, you might assume you are safe for decades. That is not how probability works. The dice are rolled fresh every year. A 1% chance each year, compounded over a lifetime, adds up substantially: over a period of around 70 years – roughly the time you might own a home, or the design life of a building – there is close to an even chance of experiencing at least one 1% AEP flood. Two such floods can also occur in consecutive years, or even the same year, as parts of NSW have seen first-hand.

So when a flood report says your land is affected by the 1% AEP event, read it as “a flood with a real, ongoing likelihood over the life of the building” – not “a once-in-a-lifetime event”.

PMF: the biggest flood that could conceivably happen

The probable maximum flood (PMF) is the largest flood that could conceivably occur at a location. It is derived from the probable maximum precipitation – an estimate of the greatest depth of rainfall meteorologically possible over the catchment – routed through the catchment to produce a flood level.

The PMF matters for two reasons:

• It defines the outer extent of flood-prone land. If your site is below the PMF level, it is technically flood-prone, even if it sits well above the 1% AEP level.
• It drives emergency planning. Councils and the SES use the full range of floods up to the PMF to think about evacuation, isolation and shelter, particularly for vulnerable land uses such as childcare, seniors housing and aged care.

Being within the PMF extent does not usually mean your floor level must be above the PMF – for most residential development that would be impractical. But it can trigger questions about evacuation routes and emergency response, and for vulnerable uses it can be a significant hurdle. The NSW Flood Risk Management Manual (2023), which replaced the 2005 Floodplain Development Manual, reinforces this whole-of-range approach to flood risk rather than a narrow focus on a single design event.

Freeboard: the safety buffer on top

Freeboard is a height buffer added to a predicted flood level to account for the things a flood model cannot perfectly capture: wave action from wind or passing vehicles, local variations in flood behaviour, blockage of culverts and bridges, and the inherent uncertainty in modelling itself.

In NSW, a freeboard of 0.5 metres above the 1% AEP flood level is typical for residential development, though councils set their own values and some apply different freeboards to different situations. Freeboard is not padding for a bigger flood – it is an acknowledgment that the predicted level for the design flood carries uncertainty, and that floors should not be set right at the waterline of a model output.

Flood planning level: the number that sets your floor

Put the pieces together and you get the flood planning level (FPL):

• FPL = selected flood event + freeboard
• Most commonly for residential floor levels: 1% AEP flood level + 0.5 m freeboard

The FPL is the minimum level for habitable floor levels in most residential development controls. The flood planning area is the land that sits below the FPL – this is the area where flood-related development controls formally apply, and it is what the flood notations on your section 10.7 planning certificate generally relate to.

Two practical notes. First, the FPL is location-specific: flood levels vary along a watercourse, so your engineer will apply the level relevant to your site, usually to Australian Height Datum (AHD). Second, different councils select different flood events for different land uses – a critical facility may be assessed against a rarer event than a dwelling, and some controls reference the PMF for specific purposes.

Hazard and hydraulic categories: not all flood-affected land is equal

Two more classification systems shape what you can do on flood-prone land.

Flood hazard categories

Hazard is essentially a function of depth multiplied by velocity. Shallow, slow-moving water is a nuisance; deep or fast-moving water sweeps away cars, undermines buildings and endangers lives. Flood studies map hazard categories across the floodplain, from areas generally safe for people and vehicles through to areas where all buildings are vulnerable to structural failure. The hazard category on your site influences everything from whether development is permissible to what structural requirements apply.

Hydraulic categories

Hydraulic categorisation describes the role land plays in conveying and storing floodwater:

• Floodway – the main flow path where floodwater moves with speed and force. Building or filling here obstructs flow and redirects it elsewhere; councils resist development in floodways strongly.
• Flood storage – areas that hold floodwater temporarily. Filling flood storage displaces water and raises flood levels for others, so fill is tightly controlled.
• Flood fringe – the remaining flood-affected land, where development generally has little effect on flood behaviour and is most likely to be supportable with appropriate floor levels.

Knowing which category applies to your site early is one of the most valuable pieces of due diligence you can do – a floodway notation changes the conversation entirely.

Mainstream flooding versus local overland flow

It also matters which kind of flooding affects your site:

• Mainstream flooding comes from rivers and major creeks breaking their banks. It is usually mapped in council flood studies, tends to be deeper and longer-lasting, and often comes with some warning time.
• Local overland flow is stormwater running across the surface when rainfall exceeds the capacity of pipes and gutters. It can affect land nowhere near a river, arrives with little or no warning, and is often less comprehensively mapped – meaning a site-specific assessment by an engineer may be needed to define it.

Plenty of urban sites in Sydney and regional NSW are affected only by overland flow, yet still carry flood-related DA requirements. Do not assume that being far from a river means the flood chapters of the DCP do not apply to you.

Why your garage can be lower than your living room

Councils commonly apply different standards to different parts of a building, and this is deliberate risk-based thinking rather than inconsistency. Habitable rooms – bedrooms, living areas, kitchens – typically must have floors at or above the FPL, because flooding of these spaces threatens people and causes the greatest damage and disruption. Garages, carports, storage areas and some non-habitable spaces can often sit lower, sometimes at or near the 1% AEP level without full freeboard, or with different requirements again, because the consequence of inundation is lower. Driveway and access levels are considered separately again, with the focus on whether vehicles and people can get in and out safely. Your engineer’s job is to weave these different requirements into a design that works on your actual block.

How flood levels are obtained for a real project

Where do the numbers actually come from?

• Council flood information requests. Most NSW councils provide adopted flood levels for a property on request, drawn from their flood studies – often the 1% AEP level, the PMF level, and sometimes hazard and hydraulic categories. This is the usual starting point.
• Existing flood studies. Council flood studies and floodplain risk management plans, prepared under the state’s flood risk management framework, contain the modelling behind those adopted levels.
• Site-specific assessment. Where council holds no adopted levels – common for overland flow paths and smaller catchments – your engineer may need to estimate flood behaviour from first principles or undertake local modelling.

Once levels are established, a flood impact and risk assessment for your DA typically covers the flood levels, depths, velocities and hazard affecting the site; the proposed floor, garage, driveway and access levels against council’s controls; the effect of the proposal (including any filling) on flood behaviour elsewhere; and evacuation or refuge arrangements where relevant.

From there, setting levels is a design exercise: habitable floors at or above the FPL, garage and non-habitable levels to the council’s specific standards, and driveways graded so that access is as flood-safe as the site allows while still meeting ordinary driveway grade limits. On tight sites, reconciling a lifted floor level with accessible driveway grades and streetscape controls is where an experienced civil engineer earns their fee.

Frequently asked questions

What is the difference between 1% AEP and a 1-in-100-year flood?

They describe the same size event – the terminology changed. AEP (Annual Exceedance Probability) expresses the chance of the flood being equalled or exceeded in any year, which for the 1% AEP event is 1-in-100 every year. The old “1-in-100-year” label wrongly suggested the flood happens once a century; in reality, over roughly a 70-year period there is close to an even chance of experiencing at least one.

Is a freeboard of 0.5 metres always required in NSW?

A freeboard of 0.5 metres above the 1% AEP level is typical for residential development in NSW, but it is not universal. Councils set their own freeboard values and may apply different figures to different land uses or flood mechanisms. Always confirm the specific requirement in your council’s development control plan or with a flood information request.

My site is below the PMF but above the 1% AEP level. Can I build?

Usually, yes – most residential controls hinge on the flood planning level (commonly 1% AEP + freeboard), not the PMF. Being within the PMF extent means the land is technically flood-prone, which can bring emergency response and evacuation considerations, and tighter scrutiny for vulnerable uses like childcare or seniors housing. A civil engineer can confirm what applies to your site and proposal.

How do I find the flood levels for my property?

Lodge a flood information request with your council – most will provide adopted flood levels and mapping from their flood studies. Check your section 10.7 planning certificate for flood notations as well. If council holds no data for your site, particularly for overland flow, a site-specific engineering assessment may be needed to establish design levels.

Get your flood levels right the first time

Contrive Consultants is a civil and stormwater engineering consultancy based in Bella Vista, working with builders, developers and rural landowners across NSW, including regional areas via remote delivery. We obtain and interpret council flood information, prepare flood impact and risk assessments, and set floor, garage and driveway levels that satisfy council controls without over-engineering your project. See our engineering services, call +61 497 848 111, email info@contriveconsultants.com.au, or contact us to discuss your site before you finalise your design.