Thousands of Sydney residents ordered to evacuate after Australia floods intensify

About 50,000 residents in New South Wales, most in Sydney's western suburbs, have been told to either evacuate or warned they might receive evacuation orders, up from Monday's 30,000, authorities said.

"This event is far from over, please don't be complacent," New South Wales Premier Dominic Perrottet told reporters. "Wherever you are, please be careful when you're driving on our roads. There are still substantial risks for flash flooding."

The latest wild storm cell - which brought heavy rains with several places receiving more than Australia's annual average rainfall in three days - is likely to ease in Sydney from Tuesday as the coastal trough moves north, the Bureau of Meteorology (BoM) said.

But the risk of flooding could remain through the week with most river catchments already near capacity even before the latest deluge.

About 90mm of rain could fall over six hours in the state's mid-north coast from Tuesday, reaching up to 125mm in some places, the BoM said.

Winds up to 90km per hour are also forecast in several flood-hit places, raising the risk of falling trees and power lines.

"We're asking people across Sydney today to please stay at home unless you really need to leave the house,” said state Emergency Management Minister Steph Cooke.

The federal government late on Monday declared the floods a natural disaster, helping flood-hit residents receive emergency funding support.

Emergency crews will continue their rescue operation on Tuesday to tow a bulk carrier ship that lost power off the coast of Sydney after tow lines broke in severe weather, officials said.

Major flooding is occurring at Windsor in Sydney's west, which is going through its third flood this year, with current water levels higher than seen in the deluge early this year, the weather bureau said.

Footage on social media showed submerged roads and bridges, while emergency crews rescued stranded people from partially submerged vehicles that became stuck in rising waters.

Nigel Myron, a Windsor resident, said he has kept an inflatable boat ready if he had to evacuate though he is looking to move back to his place once waters recede.

"At the end of the day, what can you do? It is what it is and we dust ourselves off from the ashes and rebuild after the floods have come and gone," Myron told ABC television.

(Reporting by Renju Jose; Editing by David Gregorio and Lincoln Feast.)