A severe weather warning for heavy rain, flash flooding and 55mph winds was issued along the coast of the capital and other areas in New South Wales.
Warragamba Dam in western Sydney began spilling over overnight and the peak of the overflow would be comparable to the devastating flooding in March last year, authorities said.
Although residents in some suburbs had been given the order to evacuate, emergency service officials have urged people not to wait to be told to leave, as they say the rising waters could be life-threatening.
Emergency services minister Steph Cooke said: “If you are feeling uncomfortable or unsure about your circumstances, and there is an opportunity for you to leave earlier, don’t necessarily wait for an evacuation order.”
“If you were safe in 2021 do not assume you will be safe tonight. This is a rapidly evolving situation and we could see areas impacted that we haven’t seen before.”
In the past 24 hours, emergency services have logged more than 100 flood rescues and over 3,000 requests for assistance.
Evacuation centres have opened in several areas in western Sydney while some 1,000 defence force personnel were helping to put up sandbags and knock on doors to warn of flood threats.
The weather bureau’s hazards preparation and response manager, Jane Golding, said a coastal trough of wind and rain lingering since Friday had deepened while an east coast low-pressure system formed off the mid-North coast.
She said: “That’s produced some extraordinary rainfall rates over the last 24 hours ... many locations have seen up to 200mm and some close to 300mm.”
The volume of rainfall is almost half of Sydney’s annual average. People were told to avoid any non-essential travel, including on public transport, with some roads already underwater and others at risk of sudden flash flooding.