About one-third of the country is expected to be hit by a heatwave over Christmas, with temperatures likely to reach 40 degrees Celsius or more.
The Bureau of Meteorology (BOM) says Western Australia has been experiencing a heatwave and hot air is now starting to be dragged east and south-easterly across the country.
From Friday temperatures are likely to be over 40 degrees in parts of South Australia and western Victoria, before the warm weather moves into New South Wales, Queensland and the Northern Territory.
The BOM's assistant director of weather services, Alasdair Hainsworth, says the heatwave could last for about a week.
"Unfortunately it is going to be be a fairly protracted period of hot weather," he said.
"Maybe in some of the coastal locations it won't be quite so protracted, but over inland parts of south-eastern and eastern Australia, I think we are looking at around about a week's worth of very hot weather."
Mr Hainsworth warns that Christmas travel plans may be disrupted.
"This is the first protracted heatwave of the spring-summer period over such a large area," he said.
"People should be aware that it is going to get very hot. Travel plans [may] start to be disrupted.
"We are starting to see some potential cancellations of train services in some areas due to expected buckling of lines."
"It's not that unusual at this time of year, the sun is almost exactly overhead at the present time.
"But for a period of a week or so then, to see these kinds of temperatures repeated day after day is a little unusual."
If the sweltering end to the year wasn't enough, he also says a second heatwave is possible early in the new year.
"It's highly likely that we will see a lot of that heat remain over central Australia," he said.
"Possibly into the early part of the new year we are likely to see a return to some very hot conditions."
© ABC 2013
18:15 EST After another run of very cold mornings in Victoria, there will be a few days grace for the early birds.