SYDNEY- Tens of thousands of Sydney residents were ordered to evacuate in the night on Wednesday as major flooding hit the western fringe of the city of 5 million, with torrential rain continuing to batter Australia's east coast.

Thirteen people have been killed as the extreme weather has moved from Queensland state into New South Wales (NSW), submerging town centres, washing away homes and cutting power lines, the latest four deaths recorded in the worst-hit northern NSW town of Lismore.

Rivers in Sydney's west rose rapidly on Wednesday evening, prompting the night evacuation of suburbs along the Hawkesbury River, which reached major flood levels.

Residents of Penrith, a large population centre downriver from the Warragamba dam, which began overflowing on Wednesday, were warned to prepare to leave. Authorities said inundation along the rising Nepean River at Penrith could exceed last year's flood, the worst in 60 years.

Earlier on Wednesday the bodies of two women aged in their 80s and a man in his 70s were discovered in their flooded homes in Lismore in the north of the state, and another man was found floating in the street in the town centre.

More deaths were expected as police check houses as waters recede.

State officials also told of lucky escapes, including a 93 year old woman discovered floating on a mattress 20 centimetres from the ceiling in her Lismore home by a police officer passing by in a boat.

The officer dove through a flooded window to rescue her on a "boogie board", a child's version of a surfboard, deputy state premier Paul Toole said.

The area's member of the state parliament, Janelle Saffin, 67, earlier recounted to media how she swam to safety as her home was submerged.

State Premier Dominic Perrottet, who flew over the flooded towns on Wednesday, said 17 local government areas had been declared disaster zones in an "unprecedented situation", and urged people in Sydney to evacuate if they are given the order by emergency crews.

The wild storm cell has been making its way down from Queensland state, into neighbouring New South Wales, and worsening rain was expected to hit Sydney in the early hours of Thursday.

The Bureau of Meteorology said Sydneysiders should brace for months of rain in a few hours.

"Hundreds of thousands of people have been impacted by this event," the New South Wales emergency services minister, Stephanie Cooke, told broadcaster ABC. "It is not over by any stretch of the imagination."

The disaster raised questions about how prepared the country was for being at the forefront of severe climate change, one academic expert said.

"Despite decades of warnings from scientists about climate change, Australia is unprepared for the supercharged weather that it is now driving," said Hilary Bambrick, adjunct professor at Queensland University of Technology, who led the health impacts assessment for Australia's national climate change review.

Military helicopters airlifted stranded people from rooftops, while stranded motorists and animals were rescued from a bridge in northern NSW after fast rising waters submerged both ends.

Cassie Skillings, who got stuck on the bridge along with her sister, niece and nephew, told radio station 2GB a resident rescued them in his boat.

"It was just mayhem, once shipping containers started hitting the bridge I called triple zero and they said there was nothing they could do," she said. (Reporting by Renju Jose and Kirsty Needham; editing by Jane Wardell, Robert Birsel, Alexandra Hudson)