CANBERRA -- South Australians have been warned to brace for catastrophic fire conditions and blackouts as temperatures continue to soar.

Rural Fire Service (RFS) crews engage in property protection of a number of homes along the Old Hume Highway near the town of Tahmoor as the Green Wattle Creek Fire threatens a number of communities in the southwest of Sydney, Australia, December 19, 2019. (Photo: Agencies)

Adelaide, the capital city of South Australia (SA), was one of dozens of locations across the state that experienced their hottest December day in recorded history on Thursday with temperatures soaring past 45 degrees Celsius.

The Country Fire Service (CFS) on Thursday warned of catastrophic bushfire conditions in six regions.

CFS deputy chief officer Andrew Stark told the Australian Broadcasting Corporation (ABC) that Friday's conditions were "the most dangerous ... we've faced this season" on account of extreme temperatures and high winds.

"They are the kind of conditions that are very dangerous even for our firefighters ... the kind of conditions that we see property 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."

Nullarbor in South Australia's north hit 49.9 degrees Celsius on Thursday -- the fourth hottest temperature ever recorded in Australia -- while roads melted in other parts of the state where temperatures rose above 48 degrees Celsius.

Paul Roberts, a spokesman for SA Power Networks, said that there was a "heightened risk of extended power outages" in Friday's conditions.

"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]."