A gusty change on Boxing Day will keep firefighters on "high alert" in South Australia as Adelaide faces its hottest Christmas Day for eight years.
Fire authorities have warned of elevated bushfire risk on Christmas and Boxing days after four volunteers were injured while battling a blaze in Adelaide's southern suburbs on Monday night.
Total fire bans have been issued for the Far West Coast and Mount Lofty Ranges on Christmas Day with extreme fire ratings.
On Boxing Day, total fire bans will be issued for the Mid North, Riverland, Murrayland and the upper South East.
Jonathan Fischer says Boxing Day will be windy ahead of a cool change. (ABC News)
Bureau of Meteorology meteorologist Jonathan Fischer said most central parts in the state will see the mercury hit high 30s, "even nudging 40 degrees [Celsius] for parts of the south".
"For Adelaide, we are forecasting 37C, this is going to make it the hottest Christmas Day since 2016," he said.
Country Fire Service (CFS) chief officer Brett Loughlin said fire risk in the Far West Coast would peak around the morning into the early afternoon on Christmas Day.
"For the Mount Lofty Ranges it is going to be hot all day and those winds will come and go throughout the day, that risk would be for quite a long period," he said.
"We are in the driest period on record in South Australia's history and that means all we need is heat and wind to combine to create conditions for total fire bans and for high risk fire weather."
"Unfortunately, though it's not what any of us would have wished, it's what we would see eventuate tomorrow for parts of the state on Christmas Day."
Adelaide is forecasted to have the hottest Christmas Day since 2016. (ABC News: Che Chorley)
Risk still there on Boxing Day
Despite temperatures dropping on Boxing Day, authorities warned the fire risk continued for parts of the state because of "significant winds".
Mr Fischer said Thursday would be a windy day followed by a cool change sweeping across South Australia earlier than previously predicted.
The cool change is forecasted to reach central districts, including Adelaide, around Thursday morning with temperatures dropping to mid-20s.
"But across those eastern districts would still become quite hot and windy quite early on Thursday, that's going to elevate the fire risk to extreme for a number of those eastern border districts and we'll continue to assess the timing of that change over the next 24 hours," Mr Fischer said.
"While broader fire risk is reducing through the day, it would remain windy and we're not seeing much shower activity with this change."
Mr Loughlin warned people in the eastern parts of the state to "take the risk on Boxing Day seriously" and to avoid activity that would result in "accidental ignition of fire".
"We know that the heat and these significant winds mean that any fire that starts would spread rapidly," he said.
"It's not out of the realm of possibility to see wind gusts in the 50 to 80 kilometre an hour range and that's significant.
"At that point even our firefighting aircraft may struggle to fly and may have to be grounded at parts of the day."
Peter Malinauskas says firefighters will be on high alert this festive period. (ABC News)
Premier Peter Malinauskas said South Australians living in fire danger zones should continue to be aware in coming days.
"For both Christmas Day and Boxing Day, at the very time a lot of people in our state will be relaxing, there will be others on high alert, desperately hoping their pager or beeper doesn't go off to respond to a fire that has been caused unnecessarily," he said.
Firefighters struck by tree limbs
The heightened alert comes after a grassfire was sparked on Monday at Onkaparinga Hills south of Happy Valley Reservoir.
The Country Fire Service warned residents to leave, and the fire blackened an area near Woodcroft.
Water bombers assisted the firefighting effort in what CFS state duty officer James Honner said was a "challenging" set of circumstances.
"There is a lot of population nearby," he told ABC Radio Adelaide.
"With the quick response from CFS and the assistance from the MFS as well — we had about 50 firefighters there very quickly — we were able to stop the fire from spreading.
"We've contained the fire overnight."
Four firefighters sustained minor injuries during the Onkaparinga Hills grassfire. (ABC News: Simon Goodes)
Mr Honner said no homes were damaged.
"We believe we've got one shed that was impacted — it looks like an abandoned shed inside the fire perimeter," he said.
"But otherwise we've done well to prevent impact into any other properties."
The CFS chief officer said three firefighters sustained minor injuries when being struck by falling tree branches while battling the blaze. One of them was taken to hospital for X-rays and has since been released.
A fourth firefighter rolled their ankle while working on steep terrain.
He said SA Police has confirmed the fire was suspicious, with the cause and the ignition location remained under investigation.
Firefighters will remain on the scene over the next few days.