The record books just can't stand the heat this week.

On Wednesday, the average maximum temperature from around the country was 41.9 degrees Celsius, a whole degree over the record set a day earlier.

The nation's average maximum temperature on Tuesday was 40.7C, beating the mark of 40.3C that had stood since January 2013.

The Nullarbor weather station in South Australia on Thursday broke the December location record with 49.9C at 4:54pm (ACDT) — hours earlier, Eucla (49.8C) in WA had eclipsed the mark held by Birdsville (49.5C) since 1972.

How is the average calculated?

The average maximum temperature is calculated based on a gridded data set.

The whole country is broken up into a 0.05-degree grid by longitude and latitude — which is roughly 5km x 5km — with each assigned a maximum temperature based on the weather stations around it.

An average of the grids is then calculated.

(For those really into their geography, the averaging is also weighted by area, because 0.05 degrees of longitude covers a bigger area closer to the equator)

The all-time location record for maximum temperature (50.7C at Oodnadatta in 1960) still stands.

Records in bold were set on Thursday:

State Annual record Date Town December record Date Town SA 50.7 Jan.2, 1960 Oodnadatta 49.9 Dec.19, 2019 Nullarbor WA 50.5 Feb.19, 1998 Mardie 49.8 Dec.19, 2019 Eucla NSW/ACT 49.7 Jan.10, 1939 Menindee 48.9 Dec.19, 1912 Brewarrina Hospital Queensland 49.5 Dec.24, 1972 Birdsville 49.5 Dec.24, 1972 Birdsville Victoria 48.8 Feb.7, 2009 Hopetoun 46.6 Dec.31, 1976 Robinvale Consolidated School NT 48.3 Jan.1&2, 1960 Fink 47.2 Dec.22, 1990 Jervois Tasmania 42.2 Jan.30, 2009 Scamander 39.8 Dec.20, 2015 Friendly Beaches

Melbourne is forecast to reach 44C on Friday, equal to its December record set in 1876.

Andrew Tupper from the BOM said other key forecasts included Mildura 47C, Swan Hill 47C, Warrnambool 44C and Albury-Wodonga 43C.

"The reason we talk about records is not because we like records ... it is to highlight that this is not a regular summer," he said.

"This is unusually hot, and that's why we need to pay attention to the unusually hot conditions."

Canberra, meanwhile, set a new December record on Thursday, registering 39.3C at 3:16pm (AEDT), passing the 39.2C set on December 21, 1994.

The city's all-time record of 42.2C was set in 1968 — Canberra is forecast to reach a top of 42C on Saturday.

Fire danger

New South Wales experienced another horror fire danger day on Thursday, with homes lost in the western outskirts of Sydney.

There is expected to be a lull on Friday before fire conditions peak again on Saturday.

Premier Gladys Berejiklian has declared a state of emergency that will be in place for the next seven days.

She said conditions on Saturday were not expected to be as severe as they were in mid November, but she said "we can't afford complacency".

Meanwhile, every region in South Australia is forecast to be severe and above in the fire danger ratings on Friday, as a cold front moves through bringing textbook fire conditions.

South Australia's fire danger ratings have been upgraded to six districts expecting catastrophic fire conditions Friday. ( Supplied: Bureau of Meteorology )

When is it going to end?

But the end, at least from this latest wave of heat, is in sight.

A front is moving over southern Western Australia and is expected to keep moving east over the next few days.

It is expected to bring a cool change to South Australia very late on Friday, through New South Wales on Saturday, bringing dangerous temperatures and fire weather, before the cool change.

At the moment it is looking like Christmas will be more mild, but until there is widespread rain, heatwave and fire conditions will continue.

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Longer term, the BOM's new outlook for January through March shows that while temperatures will still be up, it is looking decidedly less dire when it comes to rainfall.

So it may be time to start sniffing the air if you are up north, with the positive Indian Ocean Dipole forecast to break down in January as the monsoon eventually makes its way south.

But don't let the neutral conditions get your hopes up too much in southern Australia — your normal summer conditions are hot and dry.