Australian authorities are planning to evacuate thousands more people on Monday from flood-affected suburbs in Sydney's west, which is set for its worst flooding in 60 years with drenching rain expected to continue for the next few days.

An inundated barn is seen in the floodwaters in the residential area of Richmond suburb on March 22, 2021, as Sydney braced for its worst flooding in decades after record rainfall caused its largest dam to overflow and as deluges prompted mandatory mass evacuation orders along Australia's east coast. Photo: VCG

Unrelenting rains over the past three days swelled rivers in Australia's most populous state of New South Wales (NSW), causing widespread damage and triggering calls for mass evacuations.

"We need to brace ourselves, it will be a very difficult week," NSW State Premier Gladys Berejiklian told reporters.

Torrential rain which has submerged large swathes of NSW is in stark contrast to the weather conditions in the same regions a year ago, when authorities were battling drought and catastrophic bushfires.

"I don't know any time in a state history where we have had these extreme weather conditions in such quick succession in the middle of a pandemic," Berejiklian said.

Sydney on Sunday recorded the wettest day of 2021 with almost 111 millimeters of rain, while some regions in NSW's north coast received nearly 900 millimeters of rain in the last six days, more than three times the March average, government data showed. Authorities said around 18,000 people have been evacuated from low-lying areas of the state.

Large parts of the country's east coast will get hit by more heavy rains from Monday due to the combination of a tropical low over northern Western Australia and a coastal trough off NSW.