
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.

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.

Sydney on Sunday recorded the wettest day of the year with almost 111 mm (4.4 inches) of rain, while some regions in NSW's north coast received nearly 900 mm of rain in the last six days, more than three times the March average, government data showed.

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.

A severe flood warning has been issued for large parts of NSW as well as neighbouring Queensland.

Authorities said around 18,000 people have been evacuated from low lying areas in New South Wales.

People in parts of Sydney's northwest were ordered to flee their houses in the middle of the night as fast-moving waters caused widespread destruction. Late on Sunday, about another 1,000 people were asked to evacuate.

Television and social media footage showed fast-moving water unmooring houses, engulfing roads, breaking trees and damaging road infrastructure. Emergency services estimate the total number of damaged houses to be "in the hundreds".