Australia is basking in tradition Boxing Day weather, with temperatures hovering around 30 degrees Celsius and higher in most capital cities.

The forecast sweltering heat got off to an early start with the mercury speeding past 20C early in the day.

A heatwave expected to last until Wednesday across most of southern Australia saw Canberra reach 29C before midday with a forecast max of 33C on Monday.

Sydney and Melbourne reached 25C and 23C before midday, Adelaide was sweating through 34C heat at noon local time and Perthites woke up to 23C in the early hours.

The high heat in Western Australia has prompted total fire bans across parts of the state, with firefighters already battling 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 is playing South Africa in the Boxing Day Test, with a forecast of 32C.

Sydney will reach 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 a year’s worth of rain in just one day across parts of the Top End’s interior.

Cycle Ellie is slowly moving through the Barkly district, which which includes 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:

* Canberra, 33C max, partly cloudy.

* 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.

* Darwin, 32C max, showers with possible storms.