Australia's Bureau of Meteorology issued a severe weather warning for Tuesday.
Damaging winds, heavy rainfall, abnormally high tides and surf for all of New South Wales and the Australian Capital Territory must be expected, the bureau said late Monday local time.
"You can see rain still falling in flood areas, with more forecast for coming days," the bureau said.
Prime Minister Scott Morrison said the situation was "evolving and is extremely dangerous...This will be a very difficult week for hundreds of thousands of Australians," he said.
The bureau expects 200 millimeters to 300 millimeters of rain Tuesday and an associated flood risk. It said the floods were the worst in 20 years at least.
The bureau said around 10 million Australians in every mainland state and territory except Western Australia were subject to the weather warning - "an area similar to the size of Alaska."
Emergency services in NSW received 1,485 requests for assistance in the 24 hours to Monday night local time and completed 125 flood rescues including 70 in Sydney. The military will support NSW's flood emergency efforts from Tuesday with two search and rescue helicopters operating round the clock, The Sydney Morning Herald reported.
The Insurance Council of Australia declared an insurance catastrophe after more than 5,000 claims were lodged over the weekend. Analysts said premiums were likely to rise.
Flooding has delayed COVID vaccinations with delivery to about 50 clinics blocked while others were forced to close.
Days of continued heavy rain along the east coast have resulted in the worst flooding to hit the region in maybe half a century, some government officials said. Flooding in some areas had forced the removal of thousands of residents.
New South Wales Premier Gladys Berejiklian said Sydney was worst hit by the flooding. Berejiklian said a large part of the state's capital - particularly low-lying areas - was submerged and hundreds of homes were damaged.
"Yesterday, we were hoping it will only be a one-in-20-year event. Now it looks like a one-in-50-year event," Berejiklian said Monday.
Hundreds of people living in the city's northwestern areas were removed in the middle of the night. Berejiklian said thousands of additional people might be asked to leave if the weather doesn't improve.
Local TV and social media showed videos of fast-moving water uprooting homes, breaking trees, and destroying road infrastructure over the past few days. Officials said that "hundreds" of homes have been irreparably damaged by the flooding.
Some cities have called off school classes and at least 13 areas in New South Wales have issued flooding risk and evacuation warnings. Several roads across the state have also been rendered impassable.
Since Thursday, more than 70 centimeters of rain has drenched the northern New South Wales coast. The bureau said while the rainfall may not be as intense as in previous days it will drench already saturated land which could cause further flooding.
Meteorologists said that the rain was caused by the collision of three weather systems.
"Unfortunately what we get is tropical moisture moving into New South Wales that will spark a reinvigoration of the system east of the Dividing Range. We will see this deepen considerably Monday and into Tuesday," the bureau said.
