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More than 20,000 homes in Melbourne’s west and south-west suburbs are at new risk of flooding, according to new maps released by the city’s water authority.
The draft maps, published on Thursday morning, flag almost 15,420 homes in Hobsons Bay – which includes the suburbs of Williamstown, Altona and Spotswood – as susceptible to stormwater flooding in the event of a significant rainfall event. This marks a dramatic rise of almost eightfold from the previous number of just 1950 at-risk homes.
In neighbouring Brimbank, which includes the suburbs of Deer Park, Derrimut, Sunshine and Taylors Lakes, the number of at-risk homes has more than doubled from 4515 to approximately 11,820.
The draft maps are modelled on a “significant rainfall event” that has a 1 per cent chance of happening in any given year. It is not intended to be a general representation of flooding in the area.
The maps are released as part of a two-year plan by Melbourne Water to update its flood modelling for each of the 38 metropolitan municipalities. They will replace older flood models, which were developed using outdated techniques.
They include a scenario for the year 2100, which predicts that close to another 4000 homes in Hobsons Bay (a total of 19,410) will be under threat, alongside close to another 3000 homes in Brimbank (a total of 14,630).
In June this year, close to another 20,000 homes across Banyule and Moonee Valley in Melbourne’s north-west and north-east were also deemed at-risk.
More than 60,000 properties across the inner-north councils of Darebin and Yarra were declared flood-prone in October 2025. Later that month, another 32,000 homes were flagged as at risk in Glen Eira and Merri-bek.
As part of its modelling, Melbourne Water assigns a hazard level to properties that are potentially at flood risk. The six rating levels indicate to residents how floodwaters might behave and the potential associated risk.
The lowest level (H1) indicates minimal flooding that is shallow and limited to low-lying areas. The highest risk level (H6) warns of deep and fast-moving floodwater that is unsafe and may severely damage buildings.
In Hobsons Bay, nine in 10 homes flagged in the draft flood maps are rated either H1 or H2, meaning they are generally considered safe for both people and buildings. Almost 30 per cent of homes in the area are affected by the draft flood maps.
In Brimbank, more than eight in 10 affected homes also fall within the lowest two hazard categories. About 13 per cent of homes in that area are flagged as at-risk on the maps.
On Thursday, the maps were published on Melbourne Water’s online portal, which details how to address the risk – primarily, through preparing for stormwater. The homes flagged on the flood maps will receive a letter from the water authority asking for feedback during its consultation period on the maps.
“We encourage every resident and property owner in Brimbank and Hobsons Bay to check their address,” said Melbourne Water’s Chris Brace, executive general manager of service futures.
“For most people, the draft maps will confirm they are not at flood risk, or that their risk is lower than they might have expected. “For those who are affected, we want to make sure they have clear and practical information about what that means for their property.”
Community consultation on the draft flood maps closes on September 9.
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