Sweltering heat has been replaced on Saturday by heavy rain and severe thunderstorms in NSW, north and south of Sydney, with warnings issued for tens of thousands of residents.
A woman has died during the storms after a tree branch fell on her car while she was driving on Macquarie Pass, south of Wollongong.
A man suffered minor injuries and two other passengers were unharmed, NSW Police said.
Flights were delayed at Sydney Airport and beaches were closed as the city and the surrounding regions were lashed with rain.
Ettalong was lashed, with 93.5mm of rain in one hour, and Pearl Beach recorded 133mm in two hours earlier in the day.
Gosford recorded 56.6mm of rain in one hour; 66mm fell at Bensville; and 71mm of rainfall was recorded at Koolewong.
Very dangerous thunderstorms have been detected near Calga, Terrey Hills, Dooralong and Bundeena.
They were forecast to affect Hornsby, Berowra, Dural, Sutherland, Sydney Airport and Glenorie, according to the Bureau of Meteorology.
The weather systems were likely to drop "produce intense rainfall that may lead to dangerous and life-threatening flash flooding".
More than 880 people have called the SES for help since the severe weather began, with the majority of requests from metropolitan areas.
A very dangerous thunderstorm warning remains in place for coastal areas from Belmont to Kiama.
NSW SES spokeswoman Emily Barton told AAP that rain had been experienced "far and wide" across the state.
"We've got volunteers out responding as we speak to flood rescues and there's several incidents of leaking roofs and trees down," she said.
A strong wind warning has been issued for Sydney Harbour, Hunter Coast, Sydney Coast, Illawarra Coast and the Batemans Coast.
Southeast Queensland was also on alert on Saturday, with warnings of flash flooding, damaging winds and hail as a band of thunderstorms moves east towards the Moreton Bay area, Noosa and Gympie.
Falls up to 60mm in half an hour from the storms were reported in the Lockyer Valley and Darling Downs.
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.
19°C