Melbourne Water has released a detailed explainer to clarify one of the most misunderstood terms in flood management — the “1 in 100-year flood.” The term describes a flood event with a 1 per cent chance of occurring in any given year, known technically as a 1 per cent annual exceedance probability (AEP).
It does not mean such floods happen only once every century. In reality, several 1 per cent AEP floods could occur within a few years of each other. The standard provides a consistent method for measuring and communicating flood risk for planning, insurance, and emergency management purposes.
How 1 in 100-year flood modelling supports Melbourne Water’s work
The 1 per cent AEP standard underpins Melbourne Water’s flood mapping, infrastructure design, and emergency planning across Greater Melbourne and surrounding regions. By identifying areas at risk of inundation, these maps guide decisions on building regulations, minimum floor heights, and the siting of critical infrastructure such as hospitals, substations, and transport assets.
Melbourne Water reviews its flood maps every five years and undertakes complete updates every ten years, incorporating the latest rainfall and hydrological data. After major events, the organisation reassesses model parameters to reflect changes in catchment conditions or climate patterns.
This evidence-based approach enables the community, developers, and emergency services to make better decisions and to plan for events that could have widespread social and economic consequences.
How 1 per cent AEP floods have affected Greater Melbourne
Recent flood events across Victoria illustrate how statistically rare floods can have severe impacts. The Maribyrnong flood of 2022 was classified as a 1 per cent AEP event, inundating large parts of the suburb and demonstrating how localised rainfall can trigger significant riverine flooding.
That same year, heavy rainfall along the Yarra River caused flooding in Warringal Park and nearby areas of Heidelberg, matching the intensity of a 10 per cent AEP event. In 2016, Elster Creek in Elsternwick experienced a flood estimated between a 3 and 5 per cent AEP event, equivalent to a 20- to 30-year recurrence interval.
These examples reinforce that large floods are not hypothetical. They are part of the region’s hydrological reality — one that requires continuous monitoring and public awareness.
How Melbourne Water models and validates flood risk
To define a 1 per cent AEP flood, experts analyse long-term streamflow and rainfall data and apply numerical models that simulate how water moves through rivers and floodplains. These models account for topography, land use, and catchment conditions to predict where water will flow and how deep it will rise during an event.
Melbourne Water validates these simulations using field observations, historical flood marks, and local feedback to improve accuracy. This iterative process ensures the models reflect real-world conditions and provide confidence to both planners and the public.
How the Greater Melbourne Flood Information Program improves mapping
Melbourne Water’s Greater Melbourne Flood Information Program represents Australia’s first coordinated, city-wide flood mapping initiative. It replaces more than 1,000 local models with around 250 regional-scale simulations based on the latest Australian Rainfall and Runoff Guidelines.
The program aims to make flood data consistent, accessible, and easy for property owners to interpret. By aligning datasets across councils and agencies, Melbourne Water is helping ensure that flood-risk information supports informed decision-making, from the household to the metropolitan scale.
These maps show areas that could flood in a 1 per cent AEP event and typical water levels for that probability. While they are powerful planning tools, they do not predict when floods will occur, nor can they capture every local variable, such as debris, blockages, or unexpected rainfall patterns.
Why accurate flood maps matter for communities and planners
For households, understanding whether a property lies within a 1 per cent AEP area allows families to prepare flood plans, safeguard valuables, and respond quickly to warnings. For planners and engineers, flood maps inform the design of drainage, transport, and asset networks. Emergency managers establish and maintain evacuation routes and public safety strategies.
Melbourne Water emphasises that the purpose of its flood-mapping and modelling work is to help communities make informed decisions about flood risk. By improving the accuracy and accessibility of this data, the organisation supports safer land-use planning, stronger infrastructure design, and better emergency preparedness across Greater Melbourne.
The full explanation and updated mapping resources are available at www.melbournewater.com.au.