Record rains and widespread flooding prompted evacuations along Australia’s east coast on Saturday and threatened the most serious overflow of Sydney’s main reservoir in 30 years, officials said.

Authorities warned of potentially "life-threatening” flash floods as the torrential rains pummeled a vast coastal region already soaked by an unusually wet summer.

"It’s a very significant, record-breaking event with the rainfall that we have seen,” said Agata Imielska of the Bureau of Meteorology.

Police said hundreds of people had flocked to evacuation centres in areas north of Sydney in New South Wales state, and they expected many more to seek shelter as the rains moved south down the coast.

The Bureau of Meteorology reported flooding levels surpassing record 2013 floods along the Hastings River outside Port Macquarie, about 400 kilometres north of Sydney, though the river levels started descending later Saturday.

Towns in the area have already seen record rainfall of more than 400 millimetres since Friday morning, the bureau said.

It warned the heavy rains were forecast to move down the coast over the weekend, "potentially leading to life-threatening flash flooding” in some areas.

The flood and extreme weather warnings stretched from Port Macquarie to areas 500 kilometres south of Sydney, where residents were urged to stay indoors due to the flash flooding risk.

The Warragamba Dam, which provides much of the drinking water for Sydney, began spilling over Saturday afternoon in what experts expected to be the first significant overflow of the reservoir since 1990 although there have been smaller breaches more recently.

"We are in unchartered territory,” warned Ian Wright, a water expert at Western Sydney University, who said the rapid urbanisation of the western Sydney area around the Warragamba since 1990 meant its spillover could no longer be reabsorbed by surrounding bushland.

"The urban development adds hard, impervious surfaces, and drainage infrastructure. In heavy rain, this can rapidly generate high-velocity floodwaters,” Wright tweeted.

Authorities issued an evacuation order for the town of Picton south of the dam after the spill and were closely monitoring flood-prone areas of western Sydney.

Most of the severe flooding Saturday was along the coast north of Sydney, where television images showed city and town streets underwater, and one house literally floating down a bloated river.

Prime Minister Scott Morrison lamented the "absolutely heart-breaking scenes” of homes, towns and businesses inundated and offered troops to help with the emergency effort.

National broadcaster ABC said the rain and floods would also 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.

Emergency services reported receiving more than 3,000 calls for help and carried out about 300 flood rescues on Friday night and Saturday across the affected area.