Adelaide is forecast to be the hottest Australian capital city today with 38 degrees Celsius expected before a late shower or storm.
The forecast top is nine degrees above average.
Other areas are expected to reach the high 30s and low 40s.
By lunchtime, Adelaide’s temperature had reached 35.2C, Port Augusta had hit 39.1C and Moomba airport saw 41.4C.
On Christmas Day in 1945 Adelaide reached a top of 40.1C.
Australia’s north braces for Christmas cyclone as bushfires threaten south https://t.co/jbuhQktghA
â€” The Guardian (@guardian) December 25, 2015
While some people are making traditional roast dinners at home, many others are expected to flock to the beach to cool off.
Fire bans with a severe danger rating are in place in the state’s Northwest Pastoral, Northeast Pastoral, West Coast, Eastern Eyre Peninsula, Flinders, Mid North, Mount Lofty Ranges, Yorke Peninsula, Riverland, Murraylands, Upper South East and Lower South East districts.
People in fire ban districts are being reminded that barbecue dinners cooked on wood fires or coals are off the menu.
People caught ignoring the Country Fire Services warning face fines of up to $5,000.
Earlier this week police said officers would be patrolling in high-risk fire areas and would be keeping an eye on known firebugs.
Emergency services described the forecast conditions as the worst for Christmas Day in 30 years.