Extreme fire danger warnings have been issued for parts of South Australia as temperatures are forecast to hit the mid-40s on Saturday ahead of a change.

The Country Fire Service said there would be total fire bans across the state, with extreme ratings for the Mount Lofty Ranges, Yorke Peninsula, Lower Eyre Peninsula and the West Coast.

Several other districts have severe fire danger ratings and there will be 24-hour bans on lighting fires in the open.

Adelaide's maximum for Saturday is forecast to be 44 degrees Celsius, giving the city its fourth day of heatwave conditions.

Exceeding the 40C-mark from Wednesday through to Friday in Adelaide equalled a heatwave last experienced in 2007.

The Bureau of Meteorology said a cooler change would move through western parts of SA during Saturday and reach eastern parts by Sunday, dropping temperatures back to the mid-20s.

Fire crews have been responding to a number of fires caused by lightning strikes from a storm band moving across the state's west, mid-north and upper south-east.

They said grass and crop fires had started and there could be more problems into the evening.

Another brief burst of heat is forecast for Adelaide next week, with 39C tipped for Christmas Eve, but a partly cloudy day with a top of 29C is the bureau's tip for Christmas Day.

Two building workers affected by heat

An apprentice remains in a coma after collapsing on a work site and a second building worker has been taken to hospital because of the heat.

Travis Mellor, 17, was into just his third day of work when he was rushed unconscious to hospital in an ambulance due to a heat stroke.

He is believed to have just started a carpentry apprenticeship and was working on an eastern suburbs building site.

On Friday, a second worker on an Adelaide building site was admitted to hospital but authorities later said his condition was not life threatening.

SafeWork SA is investigating both incidents and the Master Builders' Association is urging builders to ensure they are working in a safe environment given the extreme conditions.

Adelaide's burst of heat officially became a heatwave, according to Bureau of Meteorology forecasters, just before 2:00pm on Friday when the city's maximum officially exceeded 40 degrees Celsius for a third consecutive day.

The mercury hit 40.8C, but it was as much as 10 degrees cooler at times along Adelaide's coastal suburbs because of a sea breeze.

Adelaide forecast: Saturday - Very hot, partly cloudy, 44C

Saturday - Very hot, partly cloudy, 44C Sunday - Morning shower or two, 26C

Sunday - Morning shower or two, 26C Monday - Partly cloudy, 27C

Monday - Partly cloudy, 27C Christmas Day - partly cloudy, 29C

SA Health said dozens people across the state had gone to hospitals for heat-related illnesses in the 24 hours since 8:00am on Thursday, and 21 of those were admitted.

Construction, Forestry, Mining and Energy Union (CFMEU) state secretary Aaron Cartledge said Mr Mellor was drifting in and out of coma and his family feared he could face long-term health issues.

"He's facing a real battle at the moment, some serious medical issues that are facing him, things like issues with his kidneys at the moment," he said.

"He also looks like he's got a lung infection."

The CFMEU said heatstroke was a serious condition that could cause lifelong injuries.

It said there had been cases of paralysis in Queensland workers as a result of heat stroke.

Mr Cartledge said employers needed to understand their obligations not to put workers at risk in extreme heat.

"It is utter negligence that his employer did not make a decision to stop work when the temperature had reached 41.5 degrees," he said.

"Travis is still a child, who had no idea of the danger of toiling in those kinds of conditions."

Master Builders' Association chief executive Ian Markos said the safety of workers must be a top priority.

"Heat stress is a safety issue so the main message we're saying is that there obviously is a responsibility for all employers to make sure they're working in a safe environment," he said.

South Australia bureau acting regional director John Nairn yesterday said the extremee heat was a sign of climate change.

SA Police officers take police horses for an early morning swim at Semaphore Beach. ( AAP: David Mariuz )

Extra ambulance crews put on shift

SA Ambulance paramedic Graeme Rayson said extra staff were on duty to help those affected by the heat and to make sure ambulance officers themselves were safe.

"Some of the people have been working for a number of days," he said.

A family cool off in the water at Brighton Beach in Adelaide during a four-day spell of temperatures above 40C. ( Supplied: John Richards )

"They're probably starting to feel the heat as much as the public are and we need to make sure that we look after our teams as well as the community."

Thoroughbred Racing SA (TRSA) cancelled Saturday's race meet at Morphettville due to the hot weather but Friday's twilight races went ahead as planned.

"As with every race day, the health and safety of participants is our number one priority and we have therefore decided it is the best interests of the horses, jockeys and our staff to cancel tomorrow's racing," TRSA chief executive Jim Watters said.

The Country Fire Service and Local Government Association appealed to property owners in bushfire-prone areas around the state to ensure that their blocks were cleared.

LGA president Dave Burgess said 10 per cent of properties surveyed as part of the councils' bushfire prevention strategies were not complying with bushfire safety standards and infringement notices had been issued.

"The councils will go out and inspect those properties again and if people still aren't complying then they will be issued fines," he said.

Fire prevention officers have been inspecting properties in all council districts since October.

If a property owner is found to not be complying they can be issued with an expiation notice of $315.

The State Emergency Service (SES) warned people to be aware branches could fall without warning from trees which were stressed by the extreme weather.