Areas in Victoria, Western Australia and South Australia are facing total fire bans with temperatures expected to climb.
Victoria's Country Fire Authority (CFA) has declared a total fire ban for the Mallee, North Central, Northern Country, Wimmera and South West districts on Sunday.
No fires can be lit in the open air between 00:01 and 23:59 (AEDT) on Sunday, the CFA says.
'The CFA advises people living in areas at risk of fire to activate their bush fire plan,' it said in a statement on Saturday.
The Bureau of Meteorology expects a very hot Sunday statewide, with a gusty cool change set to arrive from the southwest in the afternoon.
Milder weather is on the way for Christmas Eve and Christmas Day, with possible early showers to give way to maximum temperatures of 26C and 22C respectively, the BOM says.
Much of South Australia will also be on total fire bans, with soaring temperatures there predicted to reach similar highs.
The affected districts are Northwest Pastoral, West Coast, Eastern Eyre Peninsula, Lower Eyre Peninsula, Flinders, Mid North, Mount Lofty Ranges, Yorke Peninsula, Kangaroo Island, Riverland, Murraylands, Upper South East and Lower South East.
The CFS is urging the thousands of affected residents to ready their bushfire plans.
'If you don't have a plan to ensure your survival, leaving early before a fire starts is your safest option,' it said in a statement.
A total fire ban is in place for Western Australia's Shire of Exmouth, in the coastal Gascoyne region.
Open-air fires are banned in the district, one thousand kilometres north of Perth.
Hot work, such as metal welding and grinding, is also not permitted without an exemption.
Meanwhile, temperatures in the Pilbara region are tipped to reach the mid-40s, and inland residents in the region Shire of Ashburton face hot, dry, windy conditions.
The Department of Fire and Emergency Services is urging residents to have their bushfire plans and survival kits ready, and monitor the media for updates.