Adelaide’s ongoing severe heatwave is a first for this time of year and the “sort of signal you will see with climate change”, a Bureau of Meteorology head says.

South Australia acting regional director John Nairn said Adelaide had never experienced a sequence of four 40 degree Celsius days during December.

It comes after the city swelters through another hot night, with the mercury dropping to only 25C by about 6.30am this morning.

Temperatures peaked in the city at 41C on Wednesday, with maximums forecast to be above 40C for the next three days.

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Today’s maximum is forecast at 42C with minimum temperatures predicted to fall to 26C overnight.

“It is an unusual event,” Mr Nairn said.

“These are the sorts of signals you will see with climate change where we see the very high temperatures spread out from where they normally used to be.

“Unfortunately, this is one of the signs.”

Three districts are in total fire ban today; the Mount Lofty Ranges, Lower South-East and North-West Pastoral.

Mount Lofty Botanic Gardens will be closed to the public due to the severe fire danger rating.

About 20 people have reportedly been treated for heat-related illnesses due to the heatwave.

“With such high minimum temperatures, which are very dangerous for the elderly and the like, it is particularly dangerous,” Mr Nairn said.

“There is not much chance to actually get relief from the heat.

“As we go through consecutive days of 40 and above, it just gets hotter at night.

“It can be impossible to sleep unless you have that very expensive air conditioner on.”

Mr Nairn said a change was expected to move through the state on Saturday night through to Sunday, with maximum temperatures forecast at 30C on Sunday along with a possible shower.

He said it would still be “uncomfortable” on Sunday as heat in the environment dissipated.

Winds are also expected to increase on Saturday ahead of the change, posing a higher risk of a bushfire causing damage.