Catastrophic fire conditions have been forecast in South Australia today as the state's stifling heatwave continues into a fourth day — with temperatures hitting almost 50 degrees Celsius in some areas yesterday.

Key points: Temperatures hit almost 50 degrees Celsius in western parts of South Australia on Thursday

Temperatures hit almost 50 degrees Celsius in western parts of South Australia on Thursday Horse races scheduled for Friday evening have been cancelled due to the forecast

Horse races scheduled for Friday evening have been cancelled due to the forecast The CFS says catastrophic fire conditions are expected in parts of the state on Friday

The Bureau of Meteorology (BOM) said December temperature records had tumbled in more than a dozen locations, while some towns set all-time highs.

Parts of Adelaide sweltered through the hottest night on record, with the minimum reaching just 33.6C at 10:20pm at BOM's West Terrace site.

It was the highest minimum at the site since January 1939, when it reached 33.2C.

Senior forecaster Simon Timcke said it was a preliminary record, as the temperature could still drop, but that was very unlikely.

He said the city had a hotter night at the Kent Town site in 2009, when it only dropped to 33.9C.

The minimum at Kent Town last night 33.6C at 11:30pm.

Nullarbor was the hottest spot around the state yesterday, reaching an incredible 49.9C — the fourth hottest temperature ever recorded in Australia, overtaking Eucla in WA which reached that mark just hours earlier.

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Adelaide reached 45.3C on Thursday, its highest top temperature for the month of December since records began in 1887, and is forecast to reach 46C today.

Parts of the northern suburbs also hovered around 46C.

Ceduna's top of 48.8C was a record high, while Wudinna and Port Augusta — where roads melted in the intense heat — rose above 48C.

"McConnal Road, Alma Street, Forster Street and Cobbin Street have all shown signs of bleeding," Port Augusta City Council said in a statement.

"A contractor has been engaged to spread rocks over problem areas.

"The roads should be avoided and only used by local residents — please take an alternate route during this extreme weather."

SA Power Networks spokesman Paul Roberts warned there was a "heightened risk of extended power outages" in today's conditions.

"We have enacted detailed resource plans with multiple crews … and others on stand-by if needed," he said.

"Where conditions are confirmed to be catastrophic and there's evidence to suggest it would be wise and would help protect lives and property, we would disconnect [power to high bushfire risk areas]."

Catastrophic fire conditions in six districts

The heatwave conditions will take a dangerous turn today with extreme temperatures and high winds combining to produce catastrophic bushfire conditions in six districts.

Lower Eyre Peninsula, Yorke Peninsula, the Mid North, Mount Lofty Ranges, Kangaroo Island and Lower South East districts have all been given the highest fire danger rating.

Country Fire Service (CFS) deputy chief officer Andrew Stark said if fires break out today, they will be dangerous, fast-moving and erratic.

"They are the kind of conditions that are very dangerous even for our firefighters, from CFS, from MFS, Department of Environment and Water, the kind of conditions that we see property lost and unfortunately sometimes, lives lost," he said.

"Under these conditions [fires are] so erratic, they'll move so fast and develop so quickly, it doesn't matter how many firefighters we have, we're not going to stop the progress of the full spread of fires under these conditions."

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Mr Stark labelled today's conditions "the most dangerous … we've faced this season", and said residents in affected areas should activate their bushfire plans, if their plan is to leave.

"The safest place is to be away from areas that will see these conditions forecast, so if your plan is not to stay, you need to think about where you will go with your family," he said.

"We have a high potential for fires to break out with the effects of lightning, which will be widespread again right across South Australia.

"We will see a very gusty wind change, and even though people may start to see some relief from those winds, if we have fires burning, they will continue to be very dangerous fires for many hours after the change goes through."

Bureau of Meteorology forecaster Kylie Egan said strong winds and the risk of lightning would elevate fire danger across the state.

"We haven't really seen conditions much worse than this across the state, the wind speeds are really as strong as they can get," she said.

"The risk of lightning is certainly there, which makes it a very significant fire weather day for South Australia."

Adelaide's temperature is expected to peak in the afternoon at 46C before the cool change moves through.

Friday's twilight horse races cancelled

On Thursday afternoon, Thoroughbred Racing SA (TRSA) announced it had cancelled its Friday twilight meet at Morphettville Racecourse, despite earlier in the week saying it would still go ahead.

Thoroughbred Racing SA said Friday's twilight race meet had been cancelled due to the forecast. ( ABC News: Sowaibah Hanifie )

The organisation's chief executive officer, Jim Watters, said while TRSA had been in contact with the BOM about the forecast for Friday evening, the latest temperature predictions were higher than expected.

"While a cool change was and continues to be forecast to arrive in time for Friday's twilight race meeting, the most recent updates indicate a higher temperature than originally advised immediately prior to the cool change," he said.

"In light of this we have determined that Friday's twilight race meeting at Morphettville will be postponed, with the meeting re-scheduled to next Monday.

"As always, the welfare of our horses and participants is our number one consideration when making these decisions, and from the outset we had advised that we would monitor the situation throughout and if deemed necessary the meeting would be postponed."

The back-flip follows condemnation from animal welfare groups about TRSA's previous plan to go ahead with the meet despite the heatwave.