Temperatures have plunged by an average 10C across NSW as the heatwave that gripped much of the state starts to wane while soaring temperatures in South Australia give way to flooding rain.

Temperatures climbed into the low to mid 40Cs across central western and coastal parts of NSW on Saturday before a strong southerly buster swept up the coast, bringing thunderstorms and a welcome drop in temperature.

The sweltering temperatures are believed to have contributed to four urgent hospitalisations at a Sydney music festival where the mercury climbed to 43C.

NSW Health Minister Ryan Park said the attendees remain in a critical but stable condition while noting health services were stretched. 

“We saw around an 80 per cent increase in the number of NSW ambulance call outs and around 500 additional call outs to paramedics,” he told reporters in Wollongong on Sunday.

Clean-up efforts are under way after the evening’s southerly buster brought down trees, powerlines and many roofs.

The NSW State Emergency Service said the winds damaged several homes on the Central Coast leaving one home uninhabitable as roofs were ripped off during the storm burst.

Volunteers responded to 297 incidents across the state with Long Jetty bearing the brunt of the damage, recording 121 incidents in the past 24 hours.

Northen Zone Deputy Unit Commander Superintendent Peter Keegan said 17 storm teams are on the ground and 58 incidents are still being attended to.

“A number of properties have seen impacts, with 10 persons displaced due to the storms,” he said in a statement on Sunday.

While coastal areas experienced milder conditions following the southerly wind change, inland NSW should be on alert for widespread high fire danger, the RFS warned.

As of 3pm (AEDT) on Sunday there were about 80 fires burning across the state, several of which are yet to be contained with a total fire ban in place for parts of NSW.

In South Australia, a severe weather warning was issued for the Eyre Peninsula, West Coast, and North West Pastoral districts for heavy rainfall on Sunday.

Damaging winds are expected late Sunday and Monday morning affecting the Mount Lofty Ranges and parts of Adelaide metro, mid north and Murraylands.

Angus Hines from the Bureau of Meteorology said a long band of rain stretching from eastern Victoria across southern SA up towards the northwest had prompted the major rain warnings.

“The southern part of central SA becomes the focus point for quite heavy rain on Sunday – persistent falls, pretty much all day,” he said.

As much as 100m of rain could fall over the eastern part of the Eyre Peninsular – well more than December averages of 15 to 30mm.

“So, for a lot of places, this weekend will represent two months of rainfall, if not more than that, for this time of year,” Mr Hines said.

Victoria, Tasmania, the far south of NSW and parts of the Northern Territory can also expect high rainfall across the weekend.