Sydney's eastern suburbs and north shore will be areas of refuge from the heat, with maximums in the downtown and Terrey Hills regions reaching 29 degrees, the Bureau of Meteorology said.

Both areas can expect 31 degrees on Saturday before a late cool change arrives.

The change is likely be a modest one, with the city still expecting 27 degrees on Sunday before Monday's maximum of about 24 degrees.

There is little chance of significant rain at this point from the cooler conditions, said senior bureau forecaster Peter Gajewski.

Penrith and Richmond can expect 39 degrees on Friday and similar conditions on Saturday.

Those highs, though, will be almost moderate compared with towns in the Riverina.

Deniliquin baked in 44 degrees for a third consecutive day on Thursday and can expect 45 on Friday.

Melbourne is on course to sweat through a fourth day above 41 degrees on Friday, the first time the city has recorded such prolonged heat in records dating back to the mid-1850s.

At the Australian Open tennis tournament, 970 spectators were treated for heat-related illnesses.

The city copped 43.9 degrees on Thursday, with heat so extreme the tennis was halted, for some matches, for several hours.

More delays may happen on Friday, the bureau tipping a high of 44 degrees.

Adelaide's 44.2 degrees was the fourth day in a row above 42 degrees and a fifth is likely on Friday.

To the city's south-east, Mount Gambier broke its record maximum on Thursday.

The 44.1-degree reading beat the previous record by half a degree, said Blair Trewin, senior climatologist with the bureau.

Canberra, Melbourne and Adelaide exceeded 40 degrees on the second day in a row for the first time in more than 70 years of records, with a third day tipped for Friday, said Brett Dutschke, senior meteorologist at Weatherzone.

Every state and territory can expect a 40-degree temperature reading on Friday, Dr Dutschke said.

Adelaide's and Melbourne's maximums on Friday hinge on the timing of a cool change, its arrival potentially adding or reducing the maximums by a couple of degrees, Dr Trewin said.

Fire authorities in NSW are gearing up for a tough couple of days, with Saturday potentially more dangerous as stronger winds are forecast along with the change.

Total fire bans are in place for four regions in the state's south-east, with fire danger ratings very high or high for most of NSW on Friday.

A Rural Fire Service spokesman said ''we're still talking about Saturday'' as the day of most risk.

''There are plenty of resources ready to go,'' he said.