Thousands of people in Sydney's outer suburbs were ordered to evacuate Sunday, as Australia's east coast was hit by record rainfall and widespread flooding.

Emergency services issued the evacuation orders for several low-lying areas in the city's north-west, a day after authorities warned of potentially "life-threatening" flash floods in New South Wales state.

People had already begun flocking to evacuation centres in towns north of Sydney as torrential rains on Saturday pummelled a vast coastal region already soaked by an unusually wet summer.

In Taree, where television images showed one house floating down a bloated river, about 150 people slept in a local auditorium overnight that has previously been used a refuge for people fleeing bushfires.

Club Taree chief executive Paul Allen described the floods as a "catastrophe”, telling public broadcaster ABC that some locals had "lost everything”.

The heavy rains were forecast to move down the coast over the weekend, with the wild weather forecast to persist until as late as Thursday.

The Warragamba Dam, which provides much of the drinking water for Sydney, spilled over Saturday afternoon in what experts expected to be the first significant overflow of the reservoir since 1990.

Emergency services reported receiving more than 1,000 calls for help and carrying out about 100 flood rescues overnight Saturday into Sunday.

The rain and floods were expected to delay the already halting roll-out of coronavirus vaccines in Sydney and surrounding areas.

The country is due to begin the first major public phase of vaccine distribution on Monday although the programme has slipped behind the government’s announced timetable due to supply and delivery issues.