Floodwaters in Sydney were moving north on Wednesday, after forcing the evacuation of 50,000 people and threatening 35,000.
The city’s main reservoir overflowed, dumping what local authorities said was the equivalent to all the water in Sydney harbour into the surrounding area, continuing to deposit half as much on Monday.
The crisis was ongoing on Wednesday, although flood waters have receded in many areas.
Australia’s Bureau of Meteorology said “very warm waters off the Australian coast (21-23°C) provided extra energy and moisture contributing to the deep trough and east coast low, leading to the relative concentration of the heavy rainfall to one 24-hour period.”
In a joint briefing with state Premier Dominic Perrottet in Sydney, Prime Minister Anthony Albanese said the latest deluge is a sign “we need to act on climate change”.
“We need to show leadership and encourage that action,” Mr Albanese told reporters. “That is one of the reasons why my government is taking climate change seriously and engaging with the world to ensure that global action steps up.”
Across Sydney's western fringe, rivers broke their banks and large areas have been transformed into inland lakes, with mud-brown waters invading homes while cutting off roads and bridges.
Mr Albanese visited the affected area on Wednesday, promising to look for "long-term solutions" after multiple flooding disasters across Australia's east coast in the past 18 months.
Mr Albanese said that while "Australia has always been subject of floods, of bushfires", scientists have said climate change would make such events more frequent and intense.
"What we are seeing, unfortunately, is that play out," he said.
There were 21 flood rescues across New South Wales overnight, and on Wednesday more than 1,000 emergency service workers were in the field.
The federal government has declared a natural disaster in 23 flooded parts of the state, unlocking relief payments to stricken residents.
Many people affected have lived through successive east coast floods that struck last year and then again in March when more than 20 people were killed.
Australia's Bureau of Meteorology said the weather system was expected to move off coast later this week.
Andrew Hall, chief executive of the Insurance Council of Australia, said he expected the Sydney floods would be declared a "catastrophe" by the insurance industry.
He said 2,700 claims had been lodged by Tuesday from Sydney alone, and more were anticipated as people were able to return to their homes.
Mr Hall said there had been Aus$5 billion ($3.4 billion) in catastrophe claims made in Australia this year.
It was "untenable" for homes that had flooded four times in the past 18 months to remain in the insurance pool, Mr Hall said, adding: "We've got to stand back and ask the question, 'Have we built homes in the wrong spot?'"