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.
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 Mona Vale and Calga, moving towards Terrey Hills, Brooklyn and Spencer later in the day, according to the Bureau of Meteorology.
The weather systems are likely to drop "produce intense rainfall that may lead to dangerous and life-threatening flash flooding".
Severe thunderstorms have also been detected near Sutherland, Engadine and waters off the Royal National Park.
They were forecast to also affect Lucas Heights, Menai, Waterfall, Kurnell and Padstow, with flash flooding possible.
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 Morisset to Campbelltown.
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 is also on alert, with warnings of flash flooding, damaging winds and large hailstones as a band of thunderstorms move east towards Brisbane.
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.Â
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