People await resupply on the roof of a house along a flooded street in Sunshine Coast, Queensland, on Monday. AFP

Deadly floods swept Australia's east coast on Tuesday, stranding residents on bridges and rooftops and forcing thousands to flee their homes.

Tens of thousands of people had been ordered to evacuate their homes and many more had been told to prepare to flee as parts of Australia’s southeast coast are inundated by the worst flooding in decades that has claimed at least nine lives.

Flood warnings were in effect for dozens of areas across the states of Queensland and New South Wales, where a week-long "rain bomb" has dumped a metre (3.2 feet) of water on some areas in a week.

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Several waterways have already burst their banks or broken through levees, inundating towns and forcing residents to flee or seek safety on higher ground.

Scores of residents, some with pets, spent hours trapped on their roofs in recent days by a fast-rising river in the town of Lismore in northern New South Wales state.

This handout photo shows flood-affected properties in Ballina, New South Wales. AFP

The body of a woman in her 80s was found by a neighbour in her Lismore home on Tuesday, a police statement said. There were no details of how she died.

At least nine people have died and more than a thousand people have been rescued. Authorities have warned that more fatalities are likely.

The latest victim was a woman in her 80s, whose body was found by police inside a home in the country town of Lismore.

"She is yet to be formally identified," said New South Wales Police.

There were concerns that householders who climbed into their roof spaces through ceiling manholes could become trapped by rising waters.

A police rescue officer had saved an elderly woman from such a roof space that was almost filled with water, Lismore State Emergency Service Commander Steve Patterson said.

An Australian Army helicopter flies over flood-affected properties in the Ballina, New South Wales.  AFP

"He dived in through a window, noticed the manhole cover was open, when he checked, found a 93-year-old lady with about 20 centimeters (8 inches) left of space before the water hit the top,” Patterson told Australian Broadcasting Corp.

Dozens of cars were trapped on a bridge in the nearby town of Woodburn over Monday night with both the bridge’s approaches submerged.

Up to 50 people were rescued from the bridge early Tuesday, officials said.

"We had no capabilities to get them off in the dark so we just had to make sure that they bunkered down and we went in this morning and got them all out,” Woodburn State Emergency Services Commander Ashley Slapp said.

The flood waters are moving south into New South Wales from Queensland state in the worst disaster in the region since what was described as a once-in-a-century event in 2011.

New South Wales Premier Dominic Perrottet said there had been 1,000 rescues in his state by Tuesday and more than 6,000 calls for authorities to help.

Perrottet said 40,000 people had been ordered to evacuate, while 300,000 others had been placed under evacuation warnings.

"We’ll be doing everything ... we can to get everybody to safety and get these communities right across our state back on their feet as quickly as possible,” Perrottet said.

Government meteorologist Jonathan Howe described the amount of recent rainfall in northern New South Wales and southern Queensland as "astronomical.”

Eight of the nine deaths of the current disaster were in Queensland.

This handout photo shows flood-affected properties in the Ballina region of New South Wales.  AFP

Queensland Police Commissioner Katarina Carroll said emergency services held grave concerns for a man aged in his 70s who fell from his moored yacht in the state capital Brisbane into a swollen river on Saturday and for a 76-year-old man who disappeared with his vehicle in flood water northwest of Brisbane on Sunday.

The extraordinary rainfall comes as the United Nation’s Intergovernmental Panel on Climate Change reported this week that vast swathes of Australia have already lost 20% of its rainfall and the country’s fire risk has gone beyond worst-case scenarios developed just a few years ago.

Australia’s hottest and driest year on record was 2019 which ended with devastating wildfires across southeast Australia. The fires directly killed 33 people and another 400 people were killed by the smoke.

The fires also destroyed more than 3,000 homes and razed 19 million hectares (47 million acres) of farmland and forests.

But two La Nina weather patterns have since brought above-average rainfall to the same regions.

Lesley Hughes, an Australian academic and former lead author of the U.N. IPCC assessment reports in 2007 and 2015, said climate change was expected to overwhelm government systems such as flood responses.

"We can see that our emergency services are struggling already to cope with the floods in northern New South Wales with people stranded on roofs without food for more than 24 hours,” Hughes said.