Flooding.

What is Flood Risk?

Flood planning is an important part of the development process in New South Wales. Whether you’re lodging a Development Application (DA) or a Complying Development Certificate (CDC), councils require clear evidence that your proposal is safe, practical, and won’t worsen flooding elsewhere. Our consultancy specialises in the preparation of Flood Impact and Risk Assessment Reports that meet these requirements - helping property owners, architects, and developers gain approval with confidence.

In addition to approval support and Council policy review, Perennial also offers educational presentations and awareness training in relation to the importance of flood risk assessment and the implications of flooding on development/design.

Did you know our Director helped write the most recent stormwater management and flood risk policies for Sutherland Shire Council?

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A digital illustration depicts a city street scene with cars and pedestrians. There are streetlights, trees, and traffic signs visible, with a large building in the background under a partly cloudy sky.
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Frequently Asked Questions

Flood Basics

  • Stormwater is short-term surface runoff easily managed by gutters, pipes and drains. Flooding occurs when flow exceeds the capacity of these stormwater systems. Stormwater is frequent but minor; floods are rarer but larger and can cause significant risk to life and property.

  • Flood-prone land includes any area that may be inundated during a flood event. The most common reference point is the 1% Annual Exceedance Probability (AEP) flood, which forms the basis of the Flood Planning Area (FPA). Extreme events are measured against the Probable Maximum Flood (PMF) to plan for safe evacuation and emergency access.

    Try the Flood Levels and Risk Planning tool on LEVEE to see how these events sit across a floodplain.

  • Four terms drive most flood rules in NSW:

    • Flood Planning Level (FPL): the design flood level plus a safety buffer. New floor levels are generally set at or above it.

    • Flood Planning Area (FPA): the land below the FPL, where flood rules apply.

    • Probable Maximum Flood (PMF): the biggest flood that could ever happen. Extremely rare, but used for evacuation planning.

    • Flood control lot: a lot council has flagged (on the Section 10.7 Planning Certificate) as needing flood rules.

    If your works sit outside these flood extents, a short and simple flood assessment is often all council needs. We can tell you quickly which category your site falls into.

  • Most councils map flood-prone land and note it on Section 10.7 Planning Certificates. If your property falls within a mapped area, you'll likely need a Flood Impact and Risk Assessment Report before development approval. We can quickly review your site and confirm what type of report is required.

How Flood Risk Is Managed in NSW

  • NSW follows a set process. First, a flood study works out how floods behave in a catchment: how high, how far, how fast and how often. Next, a flood risk management study weighs up ways to manage that risk, and the chosen actions become the council's flood risk management plan.

    It is a cycle, not a one-off. After a big flood, new development or better data, the studies are updated and the cycle starts again. That is why flood levels on your property can change over time.

  • Flood studies follow rivers and creeks, not council boundaries. One river can cross several council areas, and one council can contain many catchments, each studied at a different time. Councils also update their studies at different rates.

    The rules are the same across NSW, but each council applies them with its own level of caution. That is why two similar properties in different areas can face different requirements, and why local experience with your council matters.

  • All flood modelling in Australia follows a national guideline called Australian Rainfall and Runoff 2019 (ARR2019). It sets the rainfall data and methods every model must use, and is updated as the science improves. Councils enforce it through their planning controls and study requirements, so results are consistent no matter who builds the model.

  • Freeboard is a safety buffer, typically 300mm to 600mm under the NSW Flood Risk Management Manual (2023), added on top of the predicted flood level to set the Flood Planning Level. It covers the things no model can perfectly predict, such as waves and local variations, and gives your home a margin as rainfall gets heavier with climate change. It is not over-caution; it is a deliberate allowance for real uncertainty.

  • This is one of the most common frustrations we hear. Flood rules apply at the time you develop, not the time you bought. Once a study identifies a risk and it appears on the planning certificate, council must consider that known risk in every application, no matter how long the land has been owned. Applying different rules to identical properties based on purchase date would not be fair, and approving development that puts people at known risk is not something a council can do.

    The good news: the rules almost never stop development. They shape the design, and early advice keeps the cost of complying low.

  • Flood planning protects people, property and the environment. Every development must demonstrate that it can remain safe during flooding events and won't increase flood risk for neighbours. Councils are legally required under NSW planning rules to use the best available information and ensure that new buildings and access ways are safe and properly designed.

Designing on Flood-Prone Land

  • Yes, with the right advice and design. Measures may include raising floor levels, using flood-resistant materials, or allowing water to pass beneath structures. Our reports demonstrate how a proposal meets these safety and performance requirements.

  • The rules scale with what is at stake. Bedrooms and living areas are where people sleep and shelter, so those floors generally must sit at or above the Flood Planning Level. Garages, laundries and storage can often sit lower, built from materials that tolerate getting wet.

    Below the flood level, resilient design is already standard: water-tolerant materials like masonry and fibre cement, raised power points, and layouts that let water drain away. Done well, a flood becomes a clean-up job rather than a rebuild.

  • It must not, and proving this is a core part of any flood assessment. Building or filling in a floodplain pushes water elsewhere, so councils require modelling to show any change next door is tiny, commonly around 20mm or less. Anything more is judged on what it actually does: raising the danger, flooding a neighbour's floor or cutting off their escape route will not be accepted.

    Council's flood model provides the big picture, and a site-specific model captures the local detail around your property, such as fences and sheds.

    Try the Floodplain Fill Impact Simulator on LEVEE to see why councils watch this so closely.

  • Flood models rely on assumptions, and two big ones are blockage and roughness. Blockage asks what happens when debris clogs a culvert or bridge during a flood, pushing water higher than a "clear pipe" would predict. Roughness describes how much the landscape slows water down; grass, trees and buildings all behave differently.

    Small changes in these inputs can shift predicted flood levels, so good studies test them and document every assumption. That is why choosing an experienced, transparent modeller matters. We set our inputs from measured data and national guidance, and we show our working.

  • Modern flood studies consider higher rainfall intensity, sea-level rise and changing catchments. Freeboard provides part of the buffer, and current guidance requires future climate scenarios to be modelled directly. Our assessments apply the latest methodology so your design remains compliant and resilient long-term.

  • Evacuation is the default response to flooding in NSW, led by the NSW SES. Emergency planning deliberately assumes the worst case: the biggest possible flood, with roads cut early and no rescue available. That is why an evacuation assessment can look pessimistic compared to what long-term residents remember.

    Since 2024, NSW has had formal Shelter-In-Place guidelines. Staying put in a building can be acceptable in limited cases where evacuation is not realistic and strict safety criteria are met. It must be designed for carefully, and we can advise whether it is realistic for your site.

Working With Perennial

  • Depending on your site's risk, councils may request: a Flood Impact and Risk Assessment (FIRA), a Flood Study or Hydraulic Model, and/or a Flood Emergency Response Plan. Our consultancy prepares all these documents across NSW, ensuring they align with current policy and are ready for lodgement.

  • As early as possible, typically before lodging your DA or CDC. We regularly work with homeowners, designers and planners to prepare reports that satisfy council conditions the first time. Council controls vary depending on whether a lot or development is known to be at low, medium or high risk of flooding. Early advice helps set floor levels, design drainage and avoid costly redesigns.

  • Simply send us your site address, plans or council request letter via the button below or the Contact Us request form. We'll confirm whether a flood report is required and provide a fixed-fee proposal. Reports are technically sound and ready for immediate submission with your DA or CDC.

From the first site review through to council approval
- we see it through.

Click Here to Request Free Upfront Flooding Advice