Sydney floods dramatically worsen as 50,000 on evacuation alert

Fifty thousand people may have to evacuate their homes in and around Sydney after eight months' worth of rain fell in four days.

Flooding has submerged roads and bridges, and left emergency services scrambling to rescue around 300 residents.

There have been no reported deaths so far but Premier of New South Wales Dominic Perrottet said the flooding could get worse.

"Please don't be complacent," he told people in the Australian state on Tuesday. "Wherever you are, please be careful when you're driving on our roads. There are still substantial risks for flash flooding."

A water authority spokesman said "quite an extraordinary weather event" had overwhelmed the city's Warragamba Dam, overfilling the reservoir and leading to the spillages equal to all the water in Sydney harbour.

The latest storm will probably ease in Sydney from Tuesday as the coastal trough moves north, the Bureau of Meteorology said.

But the risk of flooding could remain throughout the week, with most river catchments already near capacity, even before the latest deluge. Some regions received 800mm of rain since Saturday, eclipsing Australia's annual average rainfall of around 500mm.

Winds up to 90 kilometres 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," state Emergency Management Minister Steph Cooke said.

Battling rough seas, emergency crews continued 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, its third and most severe flood this year, according to the weather bureau.

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 an inflatable boat ready in case he has to evacuate, although 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," Mr Myron told ABC television.