Australia evacuates thousands as floods threaten Sydney

Tens of thousands of Sydney residents were ordered to evacuate as major flooding hit the western fringe of the city of five 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 powerlines, the latest four deaths recorded in the worst-hit northern NSW town of Lismore.

Rivers in Sydney's west rose rapidly, prompting the night (local time) 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, 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, 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.

State Premier Dominic Perrottet, who flew over the flooded towns, 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 tomorrow.

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.

In the hard-hit town of Lismore, Lucy Wise said the floods came much quicker and much higher than expected.

"The rain just wouldn't stop and the water was just coming up so fast" she told AFP.

She huddled at home as the waters rose through the night before grabbing her sleeping two-year-old son, cloaking him in a lifejacket and scrambling into the roof space of their house for safety.

"We were just lying there, silently, and the rain was just pouring down. I'd never heard such heavy rain in my life."

From outside neighbours watched as the house went under water.

"It was a few hours that I couldn't move. I could barely breathe. I was just taking it one breath at a time."

Ms Wise and her family were eventually rescued by boat, but authorities say the floods have already claimed the lives of 13 others in Queensland and New South Wales.

Meteorologist Ben Domensino of @Weatherzone described the current storm system as an "atmospheric river" featuring a "long area of airborne moisture that is going in one direction".

As the cleanup begins in northern areas which were hit first by the floods, many, like Mullumbimby resident Casey Whelan, predict a "long recovery" that could "take years."

Mr Whelan fled his home as the flooding worsened, but as water levels stabilised he used a kayak that had floated by and a broomstick as an oar to return. He found it "just destroyed".

Water had risen up to the height of the kitchen bench, their furniture was submerged.

"Lots of people in my street can't get flood insurance. Some insurers will quote $30,000 (US$22,000) a year... they will just be ruined. They will have no way to rebuild," he said.

Farmer James Clark said it would take weeks just to assess the damage.

"I lost tools, I lost equipment, I have got farm machinery that's underwater. I didn't get it high enough. I guessed how high the flood was going to come and got gear up, but didn't get it up enough," he said.

"After a flood it could take months before things get back to normal. It's weeks even before you can walk around without, without sort of things being too wet underfoot."