Weather watchers have said "it's almost guaranteed" all-time heat records will be broken in major Australian cities over the coming days as a scorching heatwave marches across the continent.

We could even see Australia's hottest single day in history.

Tomorrow, temperatures could exceed 20C above average in some areas with Melbourne likely to hit almost 40C. To put that into perspective, Melbourne usually only gets as high as the mid-twenties in December.

The Riverland of South Australia could breach 50C on Thursday.

Sydney's west could see 46C on Saturday while the 43C forecast in Canberra could be the capital's hottest ever temperature.

"A record heatwave is developing across southern Australia. We're expecting to break temperature records at dozens of locations across South Australia, the ACT, Victoria and New South Wales," Sky Weather Channel meteorologist Tom Saunders said.

The sheer expanse of the heat means Australia's all-time hottest day could be bested. This is measured by averaging out a day's temperatures across the nation.

"Australia's warmest day on record occurred in January 2013, when the average maximum temperature across the continent was 40.3C," Bureau of Meteorology (BOM) climatologist Dr Blair Trewin said.

"We're closely monitoring the development and progression of this heat but, based on current forecasts, we could see that record broken this week."

There are several danger days coming with elevated bushfire fears including in areas close to Sydney where blazes have destroyed homes and flames are licking the city's fringe.

Some 115 fires were burning on Tuesday morning with 59 of them uncontained.

The culprit is a trio of high pressure systems working in tandem, said Saunders.

"These three high pressure systems, on the east coast, west coast and South Coast, have created a cradle shape of high pressure that is leading to very hot air in the interior."

Saunders said it was "almost guaranteed" multiple records would fall with towns such as Port Augusta and Whyalla likely to have to rewrite their weather history books.

The BOM has said that "maximums across the board will be exceptional" for this time of year.

This searing heat is pretty much a fact of life in Central Australia right now. In Alice Springs it is forecast to top out above 45C for at least the next seven days.

Towards the coast the heat will come in pulses, with peaks on Wednesday, Friday and Saturday depending on where you are.

Starting in Perth, Wednesday could see 35C highs before a dip to the mid-twenties on Wednesday and Thursday.

Then it's back up into the mid-thirties on Saturday.

The other side of the Nullarbor has a whole lot more weather drama going on. Adelaide is in the midst of a four-day run of absolutely scorching 40C-plus highs that will see 42C on Wednesday and 44C on Friday. That will be followed by a big dip to 25C on the weekend.

Renmark, in the Riverland, could reach 48C.

Melbourne can usually expect around 25C on an average December day. Not this week. Get set for a high of 39C on Wednesday, then down to 23C on Thursday, and up again to 41C on Friday. Like Adelaide, the mercury should plummet on Saturday to just 23C.

Mildura is looking at 45C on Wednesday and 47C on Friday; Bendigo will settle around between 42 and 44C this week.

Across the Bass Strait and it will be a bonkers (for Tasmania) 33C in Hobart on Friday. In the run up to that there will be a maximum of 28C on Wednesday with more average conditions on the weekend.

Being inland, Canberra can expect all of the heat and little of the relief this week. Up to 37C on Wednesday, 41C on Thursday and then 39C on Friday.

On Saturday, Canberra could top out at 43C and nearby bushfires could lead to smoke haze. It could be the city's hottest day ever.

Sydney will see a relatively mild 27C on Wednesday and then 35C on Thursday with windy conditions fanning flames to the city's west and north west.

Down to 26C on Friday and then zooming back up to 37C on Saturday. The cool change will come on Sunday.

As ever, even hotter in the city's west. Mid-thirties for Penrith on Wednesday and then 44C on Thursday, down to 33C on Friday and then a sizzling 46C which will only help those fires.

Heading up to Queensland and it's relatively mild compared to elsewhere – even if that does mean temperatures in the mid-thirties.

On Wednesday expect Brisbane to hit 29C then 30-33C for the rest of the week with a maximum of 35C on Sunday.

In the Top End, 35-36C for the foreseeable future with possible afternoon storms and a smattering of rain.

Meteorologist Sarah Scully said severe-to-extreme heatwaves in several areas will increase the risk of heat-related illnesses.

"It's important people follow the advice of health authorities, stay up to date with the latest forecasts and warnings from the bureau and remember to check in on any vulnerable family and friends," she said.