Holidaymakers heading to coastal and inland areas of Australia's south-east after Christmas face potentially "extreme heatwave" conditions that will also elevate the already heightened fire risks.

The "very hot pool of air" that built up over Australia last week – smashing records for the nation's hottest ever days – has merely re-circulated, creating "two separate rounds of extreme heat", Blair Trewin, senior climatologist at the Bureau of Meteorology, said.

Many parts of south-eastern Australia will be seared by another round of heat to see out 2019. Credit:Alex Ellinghausen

"There's certainly the potential for further days to reach maximums above 40 degrees in the next week," Dr Trewin said.

On December 17, Australia averaged 40.9 degrees, beating a January 7, 2013 record of 40.3 degrees. A day later, the bar was raised a full degree higher, at 41.9 degrees, stunning climate scientists by the size of the margin. At least six days have topped 40 degrees on average during the current hot spell.