An evacuation warning has been issued for part of Western Sydney, as the deadly flood crisis that has hit Queensland and northern NSW moves south.

The State Emergency Service (SES) told people in Chipping Norton and Milperra, in the Harbour City's south-west, that rising waters from the Georges River "may isolate the area".

"You may be trapped without power, water and other essential services and it may be too dangerous to rescue you," the flood warning read.

It comes as 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, 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 Deputy Premier Paul Toole said residents should prepare to evacuate and the focus was on Sydney today.

"If you are asked to leave, please leave. Please do this for your own safety. Please do not contemplate on staying. Please do the right thing and don't risk your life at this point in time," he said.

The SES has warned people in North Richmond to prepare for "possible major flooding".

It says people in Windsor should prepare for moderate flooding.

NSW Premier Dominic Perrottet has urged Sydney residents not to "be complacent" about potential 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. 

BoM forecaster Dean 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.

Emergency services are already stretched across the state, receiving 8,800 calls since Monday and performing 300 flood rescues overnight.

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