Adelaide has broken the record for its longest December heatwave on file as temperatures reached 40.9 degrees Celsius just before 12:00pm.

The temperature was forecast to reach 44C by mid-afternoon, but peaked at 42.8C in the city at 4:00pm.

It was the capital city's fourth consecutive day over 40C.

Adelaide matched its 2007 December record of three consecutive days above 40C on Friday.

Port Augusta reached a top of 47.1 just before 3:00pm.

But Bureau of Meteorology senior forecaster Mark Anolak said the heatwave was now drawing to a close, with relief in sight.

"We have a cool change making its way across the western part of the state overnight tonight, probably getting to the Adelaide and the central districts in the very early hours of tomorrow morning and then the eastern districts tomorrow morning so there will be a wind change that will bring temperatures down."

Hot and dry conditions across the state have lead to a total fire ban across all regions.

An extreme fire danger rating has been declared across the West Coast, Lower Eyre Peninsula, Mid North, Mount Lofty Ranges, Yorke Peninsula and Lower South East.

A severe weather warning for parts of the state has been issued.

The CFS has responded to about two dozen call outs, most of them about grass and crop fires.

Heatstroke victim remains in hospital

One person has been discharged from the Royal Adelaide Hospital after suffering suspected heatstroke while another is still receiving treatment for the same condition.

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A 17-year-old was admitted on Friday while working on the new Royal Adelaide Hospital site and has since returned home.

Another worker from an Adelaide building site remains in hospital after being admitted on Thursday.

CFS state coordinator Leigh Miller told 891 ABC Adelaide the last time the temperature reached 44C in the city was on January 2 - the day the Sampson Flat bushfire began.

The fire in the Adelaide Hills destroyed 24 houses as well as sheds and farms as it burnt about 12,500 hectares of land across six days.

The weather bureau said Adelaide's record-breaking December heatwave would finally come to an end tonight as a band of thundery showers moves across Western Australia.

Senior-duty forecaster Matt Collopy said while the cool change was on its way, it would take a while for residents to feel the relief in their homes.

"At this stage that change looks like it moves across the western parts of our state during the latter part of this morning and then this afternoon across Eyre Peninsula and then reaches the Adelaide area very late this evening," he said.

"And it won't happen in an instant, it'll take a little while for the heat to dissipate from people's houses.

"There is some thunderstorm activity and rain developing near the western border and that will push across with this change.

"Primarily for Adelaide in the early hours of Sunday morning and then around sunrise."

Pinery bushfire relief concert cancelled

A concert that was due to take place in Kapunda to raise funds for victims of the recent Pinery bushfire has been cancelled due to the extreme heat.

The Pinery fire, which started on November 25, killed two people and burnt through more than 85,000 hectares of land.

Tens of thousands of livestock were killed, 91 homes were destroyed and the fire razed more than 300 farm sheds and outbuildings.

Concert organiser David Stokes said the committee decided it was too risky for the fundraiser to go ahead.

"The forecast was looking at 44C in Adelaide, which would be 50C in Kapunda, [with] winds of up to 50 kilometres and hour, which is a very similar situation of the day the fires actually occurred," Mr Stokes said.

"We didn't want to put anybody at risk. We don't need anybody driving around in this country, especially in this heat.

"We don't know what's going to happen and of course the CFS and everybody is on high alert at the moment and we just pray that nobody makes a mistake and we have another fire."

Mr Stokes said he hoped the concert would go ahead in early May instead.