Adelaide is on track to post a record December heatwave with four consecutive days topping 40C.
Such a stretch of extreme heat has never before been recorded in December, the Bureau of Meteorology has confirmed.
In January 2009 Adelaide had six days above 40C and four above 43C during a heatwave that lasted 16 days.
On Wednesday the mercury reached 41.1C, topped 42C on Thursday and is forecast to reach similar highs on Friday and Saturday before a cool changes sweeps through South Australia on Sunday.
The scorching conditions prompted warnings from health authorities for people to take particular care of the elderly and the very young who were most vulnerable.
SA Health chief medical officer Paddy Phillips said so far only a handful of people had presented to local hospitals with heat-related issues.
"Heat can be a killer and we don't want to see that," he said.
The SA Ambulance Service said its workload had not increased so far, but that was expected to change as the heatwave continued.
The Red Cross also began calling about 1000 vulnerable people to check on their wellbeing.
If calls go unanswered or if a person is in distress, emergency services will be contacted.
The extreme conditions are the result of a stationary high in the Tasman Sea which is pushing hot air from central Australia down into SA.
"It's ground to a stop and it's pouring the inland heat down across our state," Bureau of Meteorology acting regional director John Nairn said.
"It's sitting there and it's unrelenting."
Fortunately for the state an end is in sight, with the latest forecast models confirming the change early Sunday morning.
But Mr Nairn said the extreme conditions could return until the onset of the northern monsoon which would act to cool inland soils.
In other responses to the heat, Adelaide Metro warned the heat could damage train lines or force trains to go slow, impacting schedules and services.
The city's airconditioned central bus station was to remain open 24 hours a day to offer some people relief.
TRACKING THE TEMPERATURE IN ADELAIDE ON THURSDAY:
*6am: 25.0
*7am: 25.4
*8am: 31.8
*9am: 34.9
*10am: 37.3
*11am: 38.5
*12pm: 39.9
*1pm: 41.2
*2pm: 42.1
*3pm: 41.4
*3:55pm: 42.9 (max)
*4pm: 42.1
