IT'S going to get worse before it gets better but Australia's heatwave is set to subside with the overdue arrival on the monsoon season in the top end.

Friday is forecast to bring extreme temperatures, strong winds and dangerous fire conditions ahead of a much anticipated cool change on the weekend.

Bureau of Meteorology weather services assistant director Alasdair Hainsworth said the extended period of extreme heat affecting large parts of the nation for the last fortnight will start to ease over the weekend.

"Unfortunately high temperatures reaching into the mid-40s over eastern Western Australia, South Australia, western Queensland and large parts of Victoria and NSW are expected today (Thursday) and tomorrow (Friday)," Mr Hainsworth said.

"This extreme heat, combined with strong, gusty winds ahead of the cool change will prove particularly challenging for firefighters who have been battling blazes for two weeks now."

Mr Hainsworth said the late onset of the monsoon season had allowed hot air to build up in northern Australia.

But there is relief in sight with some cloud cover and rain expected in northern Australia on the weekend.

This will eventually flush out the hot air mass that has built up in the nation's interior, causing sweltering conditions and risky fire conditions across NSW and Victoria.

Northern Territory climate services manager Joel Lisonbee said all indicators were pointed to the arrival of the well overdue Australian monsoon.

"This year the monsoon onset is about three weeks later than we would usually expect to see it, although this is not a first," Mr Lisonbee said.

"During the summer, or northern wet season, of 1972-1973 and 1992-1993 we also saw a late onset of the monsoon, in the last week of January."

A strong, stationary mid-level ridge over eastern Australia had prevented the monsoon trough from moving onshore and contributed to the heatwave conditions.

"As the monsoon trough moves south over the continent the hot, dry air mass will be gradually replaced by a moist, tropical air mass - bringing welcome relief in the form of cloud cover, rainfall and lower temperatures across central Australia," Mr Lisonbee said.