'Rapidly evolving': dangerous wet weather lashes coast

Dangerous thunderstorms are lashing parts of Australia's east coast, forcing beach closures and prompting flash-flood warnings.

Sweltering heat has been replaced by heavy rain and damaging winds along NSW's south coast and parts of the Illawarra, Southern Tablelands and Snowy Mountains.

More than 880 people have called the SES for help since the severe weather began, with the majority of requests from metropolitan areas.

The Bureau of Meteorology warned showers and isolated thunderstorms would bring heavy rain to parts of Greater Sydney on Saturday afternoon.

A very dangerous thunderstorm warning remains in place for the central coast and mid-north coast regions, which could see isolated rainfall of up to 120mm in the next 24 hours, as well as strong winds and hail.

The bureau warns heavy falls could lead to dangerous and life-threatening flash flooding near Palm Beach and Umina.

Senior meteorologist Edward Townsend-Medlock told AAP that significant rain had fallen across the state, including 70-80mm in Blackheat in the Blue Mountains.

Within the past two hours, rainfall totals reached 100mm at Great Mackerel Beach and 83mm at Avalon.

At least 200mm of rain fell within the past 28 hours at Moruya on the far south coast.

"On the central coast, we have storm warnings out as well as a very dangerous tag on some of those storm cells," Mr Townsend-Medlock said.

"They are bringing in a huge amount of rainfall to that area, which has led to some flash flooding and a lot of activity from the SES."

"It's rapidly evolving."

NSW SES spokeswoman Emily Barton told AAP that rain had been experienced "far and wide" across the state.

She said it was important for people to take necessary precautions in the coming hours.

"We've got volunteers out responding as we speak to flood rescues and there's several incidents of leaking roofs and trees down," Ms Barton said.

A hazardous surf warning is in place for the Hunter, Sydney, Illawarra, Batemans and Eden coasts.

Damaging winds are expected for the mountainous areas around the Snowy Mountains and Southern Tablelands, with strong east to southeasterly winds.

Clean-up efforts are under way in parts of Victoria after torrential rain quickly overfilled the Wye, Kennett and Cumberland rivers in holiday hotspots along the Great Ocean Road.

The extreme event carried huge amounts of water downstream, swamping campgrounds and upending vehicles.

Multiple cars remain stranded in the surf as authorities estimate some 10 to 20 vehicles to have been lost.

More than 178mm fell in the area in six hours, with the Lorne station registering its highest 24-hour rainfall total since records began in 1884.

Disaster assistance has been announced for flood-affected communities, with funding provided to councils for emergency relief centres.

The support also covers emergency accommodation, the restoration of essential public assets and counter-disaster operations.

Communities are urged to monitor the latest warnings via the State Emergency Service and weather bureau.