Adelaide is expected to swelter through at least two more days of scorching heat, as South Australia gears up for its third consecutive 40-degree day.

The mercury is set to soar towards 43 degrees on Friday with relatively still winds of 20-25 kilometres per hour.

Still winds will mitigate the fire threat on Friday but stronger winds on Saturday are considered the biggest concern of state fire authorities.

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Fire bans were imposed across the Flinders, Mid North, Mount Lofty Ranges, Riverland and Murraylands districts on Friday, with a state-wide fire ban likely to be imposed on Saturday.

“People will need to be thinking about their fire plans,” said a Country Fire Service spokesman.

“There’s a chance of storm activity in the far north and that’s probably our biggest concern today, of lightning strikes starting fires in the pastoral lands.”

A 17-year old Adelaide apprentice is still in hospital after suffering heatstroke on only his third day on the job.

CMFEU state secretary Aaron Cartledge said workers didn’t need to risk their health by working in the extreme heat.

“The sun is up at 6 o’clock these days, getting in,” he told the ABC.

“And when the temperature reaches 37 go home.

“They lose very little production, in fact probably gain production the next day not being exposed to the heat.”

CFS said it has all its resources deployed to stamp out any fires early, and warned South Australians not to undertake any risks that may light a fire.

The safest option is to leave while it remains safe to do so.

Adelaide and many South Australian regional centres are set to post four consecutive days above 40 degrees this week, the first time that has happened in December.

Victoria is also expected to suffer through two more days of hot weather, peaking at 41 degrees on Saturday.

Total fire bans have been imposed in the state’s central region on Friday, including Melbourne, and extending to a state-wide ban on Saturday.