A heatwave expected to last until Wednesday across most of southern Australia has seen Canberra reach 29C before midday with a forecast max of 33C on Monday.
Sydney and Melbourne reached 25C and 23C before midday, Adelaide is sweating through 33C heat at 11am local time and Perthites are waking up to 23C in the early hours.
The high heat in Western Australia has prompted total fire bans across part of the state as firefighters already battle bushfires over December.
The minor heatwave is expected to bring temperatures of 27C or above to most state and territory capitals on Monday, giving Australians an excuse to put their feet up and gorge on Christmas Day leftovers.
Melburnians will have excellent conditions for a dip, or a trip to the MCG where Australia plays South Africa in the Boxing Day Test, with a forecast of 32C.
Canberra will reach 33C and Sydney 29C, Brisbane and Perth can expect the temperature to climb to 27C, while Hobart will be relatively mild at 23C, according to forecasts.
Adelaide will be a sweltering 37C and mostly sunny, and Darwin will be 32C and raining.
The Northern Territory is set for further flooding on Boxing Day as ex-tropical cyclone Ellie dumps rain and lashes the interior including Tennant Creek, Elliott, Ali Curung, Ampilatwatja, Barrow Creek and Renner Springs.
The Bureau of Meteorology warned locally intense, six-hour rainfall totals of up to 200mm could bring dangerous and life-threatening flash flooding to the Barkly district, particularly areas east of Tennant Creek.
Ex-cyclone Ellie could also bring heavy rainfall for part of western Queensland, which may cause localised flash flooding in the Diamantina and Georgina Rivers, Eyre Creek and part of the Simpson Desert.
To the south, communities along the Murray in South Australia are keeping watch with the river expected to have peaked in Renmark and due to peak downstream at Mannum in about two weeks.
Flood levees have been holding amid the deluge, but people in the Bolto shack area were told to shelter in place after damage to a road cut them off on Christmas Day.
The bureau also has a number of flood warnings in place for parts of NSW and Victoria along the Murray River and its tributaries.
Heavy fog caused travel headaches in Sydney earlier on Monday with flights into the NSW capital delayed and the city's ferry service temporarily halted.
Boxing Day forecasts:
* Sydney, 29C max, mostly sunny.
* Melbourne, 32C max, possible afternoon shower or storm.
* Brisbane, 27C max, shower or two.
* Perth, 27C max, sunny.
* Adelaide, 37C max, mostly sunny.
* Hobart, 23C max, partly cloudy.
* Canberra, 33C max, partly cloudy.
* Darwin, 32C max, showers with possible storms.
22°C