The Bureau of Meteorology warned that dry, hot conditions will create extreme fire conditions across multiple states as temperatures top 45C in parts of the country.
The heat band has been building across Western Australia for days and will extend into South Australia, New South Wales and Victoria from Tuesday.
As the mercury rises, fire dangers are expected to reach extreme levels in South Australia, Victoria, and inland NSW with total bans expected to be in place across much of the country.
The conditions are set to be worst since the catastrophic 2019-20 summer bushfires that killed more than 30 people directly and burned through millions of hectares.
"Our main concern is the three-day period running Wednesday through Friday," senior forecaster Dean Narramore said.
"We're likely to see extreme heatwave conditions over a huge part of NSW and northern Victoria, practically everywhere between Melbourne and Sydney and extending well inland.
"It is looking like … the most significant burst of heat for southeastern Australia, particularly in terms of multiple days in a row of mid-40 temperatures, since the summer of 2019-2020."
Millions of residents in NSW and Victoria are bracing for extreme heatwave conditions from Wednesday to Friday.
"We'll see temperatures really jump up to our populated parts of southeastern Australia, with even our coastal areas looking at temperatures in the low 40s," Mr Narramore said.
"That does include Adelaide and Melbourne and everywhere in between."
Soaring temperatures are expected ensure record spikes on the electricity grid as Australians crank up the air conditioning.
"There is an increased risk of unplanned outages during prolonged heatwave conditions, due to an increase in demand on the grid," Matthew Sweeting from energy supplier Ausgrid said.
The bureau defines a heatwave as three consecutive days of above-average temperatures.
The heatwave comes after another record year with 2025 Australia's fourth-warmest on record, with the temperatures 1.3C above the 1961–1990 average.
"This follows Australia's second-warmest year in 2024," Climatology Specialist Nadine D'Argent said.
"South Australia and Western Australia both recorded their third-warmest years.
"Between January and March, and between October and December, large parts of Australia experienced heatwave conditions, reaching extreme severity at times."
Australia's warmest year was 2019, when the national annual average temperature was 1.51C above average.
