Adelaide's 80-year heat record could topple this Thursday, after authorities upgraded the forecast to a 45C "horror day".

The city's highest recorded temperature stands at 46.1C, but John Nairn of the Bureau of Meteorology said that figure could be topped.

"When we go to a number like (45C) we have to be cognizant that there's two records that we've got for Adelaide," he told reporters on Tuesday.

"One on East Terrace (Kent Town) in 2009 where we got to 45.7 and the other on West Terrace in 1939 where we reached 46.1."

"Those numbers are not far from being reached on Thursday."

The official observation and forecast location for Adelaide was West Terrace between 1871-1979 and Kent Town from 1977 until 2017, when it returned to West Terrace.

Thursday will be the last of three days of SES-issued extreme heat warnings, with temperatures exceeding 37C on Tuesday and expected to hit 40C on Wednesday.

Mr Nairn said the mercury will hover in the low 30s for most of Wednesday night and dip to a minimum of 28C about dawn on Thursday.

"It does mean that we will rise to hot temperatures quite quickly on Thursday and it will be a sustained day of heat," he said.

"Thursday is going to be a horror day."

Conditions will be more extreme in the state's regional areas, with a high of 47C forecast in Roxby Downs and Port Augusta, and 46C in Port Pirie and Whyalla.

SA Health chief medical officer Paddy Phillips said the heatwave will be worse than last week's, when some parts of the state recorded four days above 40C.

"In last week's heatwave we saw 69 people present to hospitals cross the state with heat-related conditions and 31 of those were admitted," he said.

"This heatwave will be worse than last week - that's why we're warning people to be prepared, stay cool, stay hydrated, stay well."

Professor Phillips urged the public to drink plenty of water, avoid exercise or physical activity, use air conditioners and fans and look after young children and frail relatives or neighbours.

Relief from the heat is not expected until Friday, when a cool change will bring a 33C high in Adelaide.

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