Large parts of the country are sweltering through high temperatures for the first time this summer, with South Australia experiencing what is expected to be its hottest day since January 2021.
"If it does get above 40.3C, it will be hottest day since January 2021," BOM senior meteorologist Jonathan How said.
Warnings are in place in South Australia for heat, fire danger and floods, amid the River Murray flood emergency.
The Country Fire Service has also declared total fire bans for the Lower Eyre Peninsula and Yorke Peninsula.
The ban will be in place from midnight on Tuesday to midnight Wednesday.
Premier Peter Malinauskas said South Australians needed to be vigilant about extreme heat conditions.
"While it's not a catastrophic fire day, most of the state is on an extreme warning," Mr Malinauskas said.
"It's important people remain vigilant, particularly those in vulnerable areas in respect to bushfires.
"If you live next door to an elderly person take the opportunity to check in on them."
Heat forecast across Australia
The rest of the nation is set to swelter through the next few days.
"It really does feel like a proper summer across South Australia, Victoria and New South Wales," Mr How said.
"There is a heatwave right across the south-eastern part of the country.
"We are seeing temperatures reach the low 30s for the coastal parts, and the high 30s, low 40s for inland areas.
"In Melbourne today, it's hitting a top of 37Cs for those heading out to the MCG for the cricket.
"In Hobart [it is] a warm top of 30C."
Animal rescue groups have been busy responding to call-outs over the Christmas break attending to animals in distress.
RSPCA SA's Carolyn Jones said all pets should be kept away from hot surfaces including ute trays and on walks.
"People might think the concrete has cooled down, but if you can't hold the back of your hand on the surface for five seconds, then it's too hot for your dog's paws," Ms Jones said.
"Please be mindful that dogs paws are very susceptible to being burnt ... please make sure you're either walking in the cool of the morning or later in the evening."
Cool change coming
While it was a hot day on Tuesday across Australia, a cool change was set to arrive soon, according to the BOM.
South Australians were due to get some relief from midnight.
"There is a cool change on the way, it's moving through the west and expected in Adelaide around midnight," Mr How said.
South Australian SES chief officer Chris Beattie said regions impacted by the River Murray flood emergency should be prepared for rain.
"It's quite a broad part of the state we are seeing the potential for some of those falls," Mr Beattie said.
"We have redeployed a number of pumps and councils have additional pumping assets for those communities where we get stormwater issues if we do get a heavy downfall.
"We will see temperatures drop back 10 to 15C and gusty winds."
Eastern states will have to wait a few days, while Western Australia will heat up.
"Unfortunately the cool change wouldn't be moving through Victoria, Tasmania, New South Wales, until at least Wednesday morning for parts of western Victoria and then into Melbourne during the afternoon," Mr How said.
New year forecast
New year predictions are differing across the nation.
"More broadly across Australia it is looking quite warm on New Years Day," Mr How said.
"Hot day today, cool change coming through and then warming up for the first day of the new year.
"It wouldn't be dry though particularly across Sydney and New South Wales, there'll be a couple of showers."
The BOM also said people in northern parts of Australia needed to keep an eye on a tropical low moving through after Cyclone Ellie was downgraded.
"A reminder to people heading through the northern parts of the country, northern WA, the NT and northern Queensland."
