Confronted with the worst flooding in decades, Australia has ordered tens of thousands of people to evacuate their homes by Tuesday while others in parts of the southeast coast have been told to make preparations to flee.

At least nine people have died in the turmoil. 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.

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

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.

"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 floodwaters 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.

Queensland Police Commissioner Katarina Carroll said emergency services held grave concerns for a man 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 floodwater northwest of Brisbane on Sunday.

The extraordinary rainfall comes as the United Nation’s Intergovernmental Panel on Climate Change (IPCC) 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 United Nation Intergovernmental Panel on Climate Change 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.