Thousands of Australians were ordered to evacuate their homes in Sydney's northwest and elsewhere in New South Wales state as heavy rainfall flooded rivers and waterways.

People shelter under umbrellas as they walk up steps to the Opera House, with the Sydney Harbour Bridge seen in the background, on March 2. (AFP Photo/Greg Wood)

State Emergency Service Commissioner Murray Kear

Days of heavy rain after a damp summer have seen Sydney's Warragamba Dam, the main source of the city's drinking water, reach capacity for the first time in 14 years -- prompting the water to move over its spillways late Friday.

The swollen Hawkesbury and Nepean rivers were now threatening areas around Sydney's northwest.

"This is a moderate flood but it's unusual because we haven't seen water in this river system in a while," Kear said of the waters, which are likely to swamp bridges and close roads.

Further west in New South Wales, the town of Bathurst had been cut in two by floodwaters while across the state some 2,300 people had been left isolated by the surging torrents.

A severe weather warning for flash flooding was in place in large areas of New South Wales and the southern state of Victoria was also bracing for heavy rainfall.

Kear said no homes had been inundated in the past 24 hours, but he said there had been more than 40 flood rescues -- including saving people trapped in cars -- and warned people against attempting to drive through floodwaters.

Bureau of Meteorology duty forecaster Jane Golding said the rain had eased overnight, but the slow-moving band covering much of southeastern Australia meant that it would likely build during the day.

"It's quite patchy at the moment but will become more widespread and the intensity will increase," she told ABC Radio.