Roads have been blocked and at least 18 evacuation orders are in effect in the west of the city, an area hit by severe flooding in March.
“This is a life-threatening emergency,” said Stephanie Cooke, Minister of Emergency Services for the state of New South Wales, of which Sydney is the capital.
Australia is particularly affected by climate change, regularly hit by droughts, devastating forest fires, not to mention repeated and increasingly intense floods.
With further bad weather expected in the coming days, Ms Cooke described a ‘rapidly evolving situation’ and warned people should be ‘ready to evacuate’ urgently.
The Warragamba dam began to overflow in the early hours of Sunday morning, she said, well ahead of authorities’ forecasts.
In Camden, a southwestern suburb of Sydney home to more than 100,000 people, shops and a petrol station were already flooded.
Emergency services have rescued 29 people and been called more than 1,400 times in the past 24 hours.
In March, flooding caused by severe storms devastated Western Sydney and claimed 20 lives.
As the planet warms, the atmosphere contains more water vapour, increasing the chances of heavy rainfall events, scientists say. These rains, associated with other factors linked in particular to land development, promote flooding.