Emergency services have been put on high alert as forecasters predict temperatures over 40 degrees Celsius for large parts of the country.
The heatwave is expected to be of an unusually large scale - both in land area and forecast high temperatures.
Fire services in South Australia and Victoria are on high alert and residents in bushfire-prone areas are being urged to be prepared in case the worst happens.
A severe heatwave has already settled over the northern parts of South Australia and the Northern Territory's south.
The temperature is forecast to reach 46 degrees at Tarcoola in South Australia's north-west tomorrow, and 47 degrees on Monday.
At Roxby Downs in the state's north, maximums from tomorrow are expected to range between 44 and 46 degrees over a six-day stretch.
From tomorrow, Adelaide is facing 41 or 42 degrees for the next several days.
The hot, northerly winds are also spreading further into western Queensland and New South Wales, Victoria and Tasmania.
In Victoria, extreme fire conditions are forecast for Wimmera and the South West from tomorrow.
Temperatures will soar into the 40s in the state's north today and are expected to stay that high until early next week.
But there will be some relief for Melbourne and Hobart on Saturday thanks to a wind change.
Victoria Fire Services Commissioner Craig Lapsley says emergency services are well prepared for the danger ahead.
"We're moving into a really hot period for the week," he said.
"We're seeing the central Australia hot weather now moving into South Australia and Victoria. It will be around for a few days.
"Tomorrow we will see an extreme fire rating in SA and a severe rating for the Murray River, central Victoria, Melbourne and the west part of Gippsland."
© ABC 2013
13:56 EDT Like a large area of southeastern Australia, Victoria has been been experiencing a chilly run, as much as four-to-ten degrees below average but is now thawing out.