The Bureau of Meteorology is predicting a maximum of 38 degrees for the city, 39 for Parramatta and 40 for Penrith before a cool change brings relief by early evening.

The sweltering conditions should subside on Saturday, but only for a day as the arrival of another trough draws more of a massive hot air mass sitting over central Australia into the south-east of the country.

The mercury is likely to climb back to the mid-30s for Sydney on Sunday, with 41 degrees predicted for regions further west, such as Penrith.

"It's pretty significant [heat] for November, that's for sure," Rob Sharpe, a meteorologist at Weatherzone, said.

Sydney's top on Friday is unlikely to exceed the the 40.4 degrees registered almost four years to the day but will still be about 14 degrees above average for the month.

Fire authorities on Thursday extended the range of Australia likely to experience an above normal fire season this summer. Virtually all the populated centres stretching along the coast from Adelaide to Brisbane can now expect a busier than usual bushfire season, according to the Bushfire & Natural Hazards Cooperative Research Centre.

A heatwave projection service provided by the weather bureau projects a three-day heatwave from Friday stretching from north-eastern Victoria all the way to Darwin, covering almost all the regions to the east.

For towns such as Bathurst, west of the Blue Mountains, the coming burst of heat is likely to be the sixth heatwave since the start of October. That gauge is based on at least three days in a row during which the maximums are in the top 10 per cent of records for those days, according to the Scorcher website produced at the University of NSW.