Rain-swollen rivers spilled mud-brown waters across swathes of Sydney on Tuesday, swamping homes and roads while forcing thousands to flee.

The authorities have now instructed about 50,000 people to evacuate and another 28,000 to prepare to escape the rising waters in New South Wales, officials said.

Emergency workers carried out 142 flood rescues in New South Wales over 24 hours, they said, with the support of 100 troops deployed to the state.

Australia has been at the sharp end of climate change, with droughts, deadly bushfires, bleaching events on the Great Barrier Reef and floods becoming more common and intense as global weather patterns change.

Higher temperatures mean the atmosphere holds more moisture, unleashing more rain.

With much of the ground already sodden, the water rose rapidly in worst-hit areas and was soon lapping around the walls of some homes in the western Sydney suburbs.

"All of a sudden it just came up so quick," said resident Gordon Lee after parts of his western Sydney suburb of Shanes Park were engulfed overnight.

"We didn't even have time to take anything, just got our pet dogs and went out to higher ground up the street there," he said.

Lee said he had retired from farming about 15 years ago when flooding was less frequent. "I see younger people move in trying to farm here but they are getting a hiding," he said. "They are losing everything."

Meteorologists predicted the weather front would move northwards along the east coast after dumping rain on Sydney for four days.

"Sydney is not out of danger, this is not a time to be complacent," State Emergency Services commissioner Carlene York told a news conference. "It's risky out there."

The federal government has declared a natural disaster in 23 flooded parts of New South Wales, unlocking relief payments to stricken residents.

Many people affected have lived through successive east coast floods that struck in 2021 and then again in March this year when more than 20 people were killed. (AFP)