Sydney is bracing 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.

Emergency services ordered people living in low-lying areas on the city's north-western fringes to flee to safety, as authorities warned of a potentially "life-threatening" situation in New South Wales state.

It came after the Warragamba Dam, which provides much of the drinking water for Sydney, spilled over yesterday, causing officials to warn the downstream Hawkesbury River was expected to peak at levels not seen since 1961.

"It is one of the biggest floods we are likely to see for a very long time," said an official from Bureau of Meteorology.

Floodwaters had already risen in several areas, prompting mass evacuations, and officials said another 4,000 people could be told to leave their homes in the coming days.

Authorities urged residents to heed the "dozens" of official warnings in place across affected areas, with the State Emergency Service assistant commissioner saying those in evacuation zones "must leave immediately".

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

State Premier Gladys Berejiklian said the region was experiencing a "one in 100 year event" and a national disaster had been declared.

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 as 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 Bureau of Meteorology said the wild weather was forecast to re-intensify north of Sydney tomorrow before easing later in the week.

Conditions were "going to be treacherous yet again," a senior climatologist at the bureau said.

Rainfall records were forecast to continue tumbling in the coming days, she added.

Emergency services reported receiving more than 7,000 calls for help and carrying out about 650 flood rescues since Thursday, with reinforcements being called in from other states.

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

Australia is due to begin the first major public phase of vaccine distribution tomorrow although the programme has slipped behind the government's announced timetable due to supply and delivery issues.