A broad area stretching across much of southern Australia is set to experience the hot weather, with temperatures generally 10C to 14C higher than usual for this time of year, the Bureau of Meteorology said.
The mercury is expected to soar above 40C in many parts, and Mildura in Victoria's northwest, could see maximum temperatures stay above 40C for four days in a row for the first time in 90 years.
"Summer is well and truly here, with warm to hot conditions developing across southern Australia to finish the year of 2018," meteorologist Jonathan How told reporters on Wednesday.
"We are seeing severe to even extreme heatwave conditions develop across quite a wide area of southern Australia, stretching from Western Australia in the Pilbara district, through South Australia into southern New South Wales and Victoria as well."
South Australia and Victoria will face an elevated fire risk with total fire bans declared for Thursday in Victoria's Mallee and Wimmera regions.
Victoria's health department has also issued a warning the conditions may lead to increased heat-related illnesses.
"Extreme heat can affect anyone. Pregnant women, people with a pre-existing medical condition, people aged over 65 and children and babies are most at risk," the department said.
"The heat isn't unusual for this time of year, but what is unusual is how protracted and extended the heat is," Mr How said.
Canberra could record temperatures above 35C for five consecutive days, and Western Sydney can expect an extended period of above 40C.
Marble Bar in Western Australia is set to hit 49C on Wednesday and might see a run of eight days above 45C for the second time this month.
Temperatures are expected to hit 35C in Melbourne and 41C in Adelaide in the coming days, but should cool over the weekend.
Australians have been warned to expect similar extreme conditions over the coming months.
"This is the first month of summer, so there is a long way to go for much of the country," Mr How said.
"At the moment the bureau is forecasting a warmer and drier summer than normal, so this could really just be a taste of things to come."
