Flood evacuation orders were issued for a swathe of suburbs across Sydney's west, south-west, and north-west on Wednesday night, as heavy rain pelts NSW.
The Nepean, Georges and Hawkesbury rivers are rising rapidly and the State Emergency Service (SES) told people in specific parts of more than 20 suburbs to leave.
There was particular concern for areas around Richmond and Windsor, in the city's north-west, where the Hawkesbury River was expected to get as high as 14 metres.
The east-coast low hitting NSW also meant major flooding on the Nepean River overnight.
On Wednesday afternoon, the Bureau of Meteorology's Dean Narramore said a "dangerous situation was evolving" in the Hawkesbury-Nepean flood plains and water would likely reach similar heights to flooding in March 2021.
NSW Deputy Premier Paul Toole said it would be a "tough night" in Sydney and warned people to monitor evacuation orders.
"This is to ensure that you're not putting your life at risk or even putting our SES or emergency service [personnel] at risk."
SES Commissioner Charlene York urged people in flood-prone areas to take the situation seriously.
"We've lost lives through this event and we don't want to lose any more."
Ms York said the SES website was having intermittent issues "due to unprecedented traffic" and messages would also be dispersed on social media.
People in the Blacktown and Liverpool areas had to be rescued by SES floodwater teams as early as the afternoon.
"Jobs are being prioritised and we are only responding to life-threatening or critical jobs at this time," a spokesperson from the Liverpool team said.
"Please never enter floodwater."
The Bureau of Meteorology (BOM) issued a swathe of severe weather and flood warnings for hundreds of kilometres of NSW's coast, from above Newcastle to Moruya Heads.
The wild weather has inundated homes and businesses in south-east Queensland and northern NSW, killing 12 people and significant numbers of animals.
Sydney's main water source, Warragamba Dam, began spilling at 3am Wednesday, which authorities have warned will have a flow-on effect for communities along the Hawkesbury and Nepean rivers, in Sydney's west and north-west.
NSW Premier Dominic Perrottet has urged Sydney residents not to "be complacent" about the flooding after seeing the devastation in the north of the state.
"If they look up [to northern NSW] and see the challenges, they're aware we could see the same issues in metropolitan Sydney and further down south by the end of the week," he said.
"I've heard first-hand from the SES that people don't follow evacuation orders in certain circumstances. What we're seeing is an unprecedented event.
Thousands of residents were evacuated the last time the dam overflowed in March 2021, when more than 250mm of rain was recorded over three days.
Mr Narramore said about 50mm to 150mm would hit some parts of Sydney, where the ground was "already saturated".
"The water has nowhere to go except for creeks, rivers, streams, eventually dams and major river systems through eastern parts of New South Wales," he said.
