The entire coast of New South Wales has been alerted, and troops and hundreds of volunteers have been dispatched.
Roads and bridges were cut, cars and signs were flooded, livestock were approved, and schools were closed.
Prime Minister Scott Morrison has told lawmakers that there is still a lot of danger and that weather officials say it is “far from over”.
New South Wales (NSW) Premier Gladys Berejiklian said no deaths had been reported – “a miracle given to us through the past”.
An estimated 18,000 people have been evacuated from the area at present, Australia’s population of eight million.
What is the latest forecast?
The heaviest rainfall in the worst-affected province of NSW is expected to continue on Tuesday night, as two major weather systems collide. Areas on the south coast may be the worst hit at this time.
A tweet released by the Australian Bureau of Meteorology Bureau said an area as large as Alaska – from the NSW coast to the southern Northern Territory – is now affected by weather warnings.
Chris Fawkes of BBC Weather explains why New South Wales was hit by such a flood
In all, 10 million people are subject to warnings in all countries and territories except Western Australia.
The low-lying climate system that has been flooding the NSW coast for days has been met by another inland-east climate system.
The office reported that “it is raining, strong winds, damaging waves and unusually high waves” in New South Wales on Tuesday.
“We may have been away for days but unfortunately this situation is not over,” the department wrote on Twitter.
Its flood manager, Justin Field, said: “I have been a flood monitor in office for 20 years and this is probably the worst flood I have ever experienced and I was forced to predict.
“We have a flood mirror covering all the way from the Queensland border to the Victoria border.”
Prime Minister Morrison told MPs on Monday: “Across the NSW, the first 1,400 respondents committed more than 700 floods and responded to more than 7,500 requests for assistance.”
He added: “This is an ever-changing and very dangerous situation.”
The dogs are being transported to a safer location on a Sydney ferry
Some places have seen about a meter in the rain.
But the office predicted that it would take a break, posting on Twitter: “Most of the New South Wales areas will see the practice early Wednesday as there is a dry wind entering the region.”
What has been the damage so far?
Rainy days have created rivers and dams overflowing in Sydney – the country’s capital – and southeastern Queensland.
About 200 schools in NSW will remain closed on Tuesday as heavy rains arrive.
The order requires people to seek shelter with family or friends away from flood-affected areas, or in a special facility. The warning tells people to prepare for the exit order.
About 15,000 people have been evacuated from the Mid-North Coast and another 3,000 from Sydney, officials said.
The areas around the Colo and Hawkesbury rivers are of great concern. They have seen the worst floods since 1961.
Scott Donnellan, a Presbyterian minister in Port Macquarie, told the BBC that some people had contacted you when the evacuation orders arrived but they had to “dispose of the ship” when the water reached the waist.
He said the public was pulling together but was expecting “one last punch” from heaven.
Among the incidents that took place:
The Warragamba Dam, Sydney’s main water source, was demolished for the first time in five years. It reached its highest daily output on Sunday by 500 gigaliters – equivalent to the capacity of Sydney Harbor
A 37-year-old woman was delivered by helicopter after being swept away by floodwaters in the Nepean River
Video on social media showed spiders infested with spiders and walls on the west side of Sydney as they tried to escape the rising water
At the North Shore of Port Macquarie, a resident was caught dragging a stingray on his former machine
On Sunday a young couple saw their house north of Sydney being swept away by a catastrophic flood on their wedding day
Prime Minister Morrison has provided a single state grant of A $ 1,000 ($ 775; £ 560) per eligible adult and A $ 400 per child eligible for those most affected by the floods.
From fire to floods
The heavy rains and floods in eastern Australia are very different from last year, when many of the same areas were burned to the ground and destroyed by drought.
This side of the continent is currently battling the climate of La Niña, which often brings heavy rain and tropical storms in summer.
Two of the three rainy years in Australia were recorded during La Niña events. Normally La Niña sees a 20% increase in moderate rainfall from December to March in eastern Australia.
Scientists say that climate change also reinforces the influence of La Niña, and makes climate patterns more stable.