Inland areas, such as in the Riverina, will approach 45 degrees on some days during the spell. Towns on the South Coast that have had a mild summer so far will begin to empty of holidaymakers just as things heat up.

"It's pretty unfortunate timing" for those heading back to work and school, Mr Domensino said.

Compared with last summer's bouts of excessive heat, bushfires and smoke, this summer has felt on the cool side even if temperatures are close to average for much of the state. It's the switch to sweltering conditions over multiple days that is part of the prompt by health authorities for people to take extra care.

“Protect yourself during the heatwave by postponing or rescheduling your outdoor activities," said Adi Vyas, acting director of Environmental Health with NSW Health in a statement issued on Thursday.

"Reduce the impact of heat by avoiding being outside during the hottest part of the day; keeping
well hydrated with water, and looking after vulnerable neighbours and relatives," he said.

Heat is often described as the silent killer, causing the premature death of more Australians than other natural causes.

“Hot weather puts a lot of strain on the body, including dehydration, and can make underlying health conditions worse,” Dr Vyas said, adding that people over 75, those with chronic medical conditions and people living alone were particularly vulnerable.

Mr Domensino said twin low-pressure systems off north-western NSW, both of which may become tropical cyclones in the next couple of days, were factors in stoking the furnace over central Australia.

Big bushfire events in south-eastern Australia, such as Black Saturday in Victoria in 2009, were preceded by heatwaves linked to cyclonic activity off the country's north-west. The spinning systems push away air in the upper atmosphere that works to enhance the development of heat over the country's south-west, he said.

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One of the possible cyclones could cross the Pilbara coast as a category 2 strength system on Saturday morning, while the other tropical low is much further to the west, near the Cocos Islands. One will be named Lucas and the other Marian, depending on which forms first.

Climate change, meanwhile, has pushed Australia's background temperatures about 1.4 degrees higher since 1910, the Bureau of Meteorology said. That means heatwaves, when they occur, tend to be more intense and last longer.

Places such as Penrith have endured scorching periods like the coming weekend several times in recent years. The suburb near the foothills of the Blue Mountains copped three consecutive days above 40 degrees last summer, and two such bouts during the two previous summers of 2017-18 and 2018-19, Mr Domensino said.

Bushfire risks will also pick up in some regions with the heat, although good rains in recent months and the forecast light winds will moderate the threat.

Greater Sydney, for instance, will have a steady "high" fire danger rating through to Monday, while much of the state's south will rise to "very high".

The southern Riverina is the exception, with the fire threat reaching "severe" on Monday.

With the extra warmth likely to send people flocking to the beach, Surf Life Saving NSW expects to be "extremely operationally busy" over the next few days, Steve Pearce, the group's chief executive, said. The organisation has already effected more than 1000 rescues since Christmas, he said.