Sydney braces for more heavy rains and flash floods as thousands flee homes

Fresh evacuation orders were issued for thousands of Sydney residents on Monday after relentless rains flooded several suburbs in Australia's largest city, with officials warning of more wild weather over the next 12 hours.

An intense low-pressure system off Australia's east coast is forecast to bring more heavy rain through Monday across the southern regions of New South Wales even as several places in the state were hit with about a month's rain over the weekend.

About 100 millimetres of rain could fall in the next 24 hours over a wide swathe of regions in New South Wales, from Newcastle to the south of Sydney, a distance of more than 300 kms, the Bureau of Meteorology said.

"We are expecting the rain to pick up again from this afternoon," Jonathan How, BoM meteorologist said.

More than 200 mm of rain have fallen over many areas, with some hit by as much as 350 mm since Saturday. 

The wild weather could trigger flash floods and landslides, with river catchments already near full capacity after the La Nina phenomenon, typically associated with increased rainfall, dominated Australia's east coast over the last two years.

Warragamba Dam, Sydney's major water supply, began to overflow well ahead of projections on Sunday.

About 70 evacuation orders are in place in Sydney as authorities urged people to leave their homes before getting stranded without power.

As tens of thousands face evacuation, frustration swelled in North Richmond and Windsor in Sydney's west after floods submerged homes there for the third time this year.

"We are over it. We are so over it. (This) is a bit much for of us," a flood-hit Windsor resident told ABC television.

Federal emergency management minister Murray Watt has offered more troops and said on Monday the government has activated the satellite emergency management system to help with the flood relief efforts.