The last time when the country touched the 40.3 degrees C mark was on January 7, 2013, said the Bureau of Meteorology (BoM) on Wednesday.
Taking the average of maximum temperatures across the country is the most accurate measure of a heatwave, the BBC reported.
The record comes as the nation battles a severe drought and bushfire crisis.
Forecasters had predicted the most intense heat would come later in the week, meaning the record could be broken again.
As hundreds of fires rage, Prime Minister Scott Morrison has been slammed for his response to the natural disasters and his government's climate policies.
Australia heated up this week as a mass of hot air swept east across the continent, with meteorologists forecasting "severe to extreme heatwave conditions".
Several individual heat records for towns and cities have already been shattered. On Tuesday, places across the nation's centre recorded temperatures above 45C.
At the start of the week, Perth, the capital of Western Australia, recorded three days in a row above 40C - a record for December.
December heat records are set to be smashed, and emergency crews including firefighters, ambulance and paramedic services are on alert, with total fire bans issued for NSW and South Australia, reported 9 News.
( With inputs from IANS )