Tuesday was the hottest day on record with an average across the country of 40.9 degrees Celsius, preliminary results show.
The Bureau of Meteorology said the average temperature beat the previous record of 40.3 degrees Celsius recorded on 7 January 2013.
Southern and central Australia sweltered as temperatures reached eight to 16 degrees above average.
Tuesday's record could soon be eclipsed though - with temperatures expected to exceed 40 in parts of Australia until the end of the week.
A dog cools off under a shower during the heat wave. Source: AAP
Adelaide is facing a four-day heatwave, with an expected peak of 44 degrees Celsius on Friday.
Victoria is also predicted to record its hottest December day on Friday if northwest towns such as Mildura and Swan Hill reach a forecast 47 degrees Celsius.
Melbourne hit 39.7 degrees Celsius just before 3pm on Wednesday, while the state's northwest is expected to crack the mid-40s.
Uncomfortable and dangerous conditions
Fire weather warnings are current for parts of Western Australia, much of South Australia and the Wimmera region of Victoria.
A state-wide fire ban remains in place in New South Wales with daytime temperatures for the next four days expected to be 14 degrees Celsius above average for this time of year.
The NSW premier has warned the state's firefighters will take on an "enormous challenge" in coming days as temperatures rise and winds further complicate dangerous bushfires.
NSW Rural Fire Service Commissioner Shane Fitzsimmons has cautioned the public, warning fire behaviour will be "erratic and significant."
The heat will start moving through New South Wales on Thursday and conditions will worsen on Saturday,” he said.
A statewide total fire ban is in place from Wednesday morning through to midnight on Saturday.
The Bureau of Meteorology warns smoke will also remain an ongoing hazard for residents across New South Wales and the nation's capital.
Authorities are reminding people to take care in the scorching summer heat and drink plenty of water.