The New South Wales minister for emergency services described the floods as a "life-threatening emergency."
Authorities in the Australian city of Sydney ordered thousands of people to evacuate their homes on Sunday, after the outskirts were hit by torrential rains.
Roads have been blocked and at least 18 evacuation orders are in place in the west of the city, an area hit by severe flooding in March.
"This is a life-threatening emergency," warned Stephanie Cooke, minister for emergency services in New South Wales, of which Sydney is the capital.
Australia is particularly hard hit by climate change, with regular droughts, devastating forest fires and repeated and increasingly intense flooding.
With more bad weather expected in the coming days, Ms Cooke described a "rapidly evolving situation" and warned that people should be "prepared to evacuate" places urgently.
The Warragamba dam began to overflow in the early hours of Sunday morning, she said, well ahead of the authorities' forecasts.
In Camden, a suburb southwest of Sydney home to more than 100,000 people, shops and a petrol station were already flooded.
Emergency services rescued 29 people and were called more than 1,400 times in the last 24 hours.
In March, flooding caused by heavy storms devastated western Sydney, killing 20 people.
As the world warms, the atmosphere contains more water vapour, increasing the risk of flooding and heavy rainfall events, scientists say. This rainfall, combined with other factors such as land-use planning, can lead to flooding.