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Sweltering heat has been replaced by heavy rain and damaging winds along NSW's south coast and parts of the Illawarra, Southern Tablelands and Snowy Mountains.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

NSW SES acting assistant commissioner Sonya Oyston said it was important people took necessary precautions and did not become complacent.

"We're starting to see many catchments become wet and we have already seen the impacts of flash flooding right across the coast and ranges," she said.

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

NSW Police urged people to consider staying out of the water and avoid walking near surf-exposed areas.

Rock fishers should avoid coastal rock platforms exposed to the ocean and seek a safe location sheltered from the surf, police said.

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

Meanwhile, clean-up efforts are under way in parts of Victoria after torrential rains 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 178 millimetres of rain fell in the area in six hours, with the Lorne station registering its highest 24-hour rainfall since records began in 1884.

Communities are urged to stay updated on the latest warnings via the local State Emergency Service and weather bureau.