The Bureau of Meteorology expects Victoria to experience the worst of the currents heatwave, with Swan Hill and Mildura in the state's north likely to reach 47oCelsius.
Mildura could break Victoria's heat record for a December day.
How hot will it get?
In Melbourne, the Bureau of Meteorology says temperatures in the CBD are expected to rise to 44oC, which would be the city's hottest day since the Black Saturday bushfires. Some of the city's suburbs are forecast to reach 45oC.
Meteorologist Dean Narramore says almost all of the state will be very hot on Friday. Narramore says a cool front should help south-western parts of Western Australia, with temperatures forecast in the high 20s.
Tasmania should also be cool, as well as coastal New South Wales, forecast to benefit from a southerly change.
"Pretty much everywhere inland, away from the east coast, and Victoria and South Australia, will be looking at temperatures in the mid to high 40s."