Adelaide and most of South Australia will swelter through a hot Christmas Day, but authorities are more concerned about conditions on Thursday when the bushfire risk will peak.
Some northern centres could experience record highs this week with temperatures pushing into the mid-40s.
Adelaide is expected to have a top of 35C on Christmas Day, but the mercury will climb to 41C by Thursday when rising winds will add to the fire threat.
"Those will be challenging conditions," Country Fire Service state duty commander Daniel Austin said on Monday.
"Forty degrees on any day is a challenge, but add the strong winds that are forecast, that potentially sees fire move fast through the landscape.
"People need to be well aware of where they are, what's around them and what they should do should a fire break out."
The CFS was dealing with one significant fire on Monday after harvesting activity caused a blaze at Mitchell on the lower Eyre Peninsula.
A watch and act message was in place for locals with the fire considered a threat to lives and property.
Ground crews were on the scene along water bombing aircraft.
Bureau of Meteorology senior forecaster Kylie Egan said the heat across SA would peak on Thursday ahead of a trough moving through and lowering temperatures.
The mercury was expected to rise again by Sunday but current modelling suggested a top of around 28C in Adelaide for New Year's Eve.
Ms Egan said conditions forecast over the next week were suggestive of a heatwave, especially in the north where temperatures would be most severe.
