Sweltering summer conditions in Sydney have been made alarmingly worse by choking smoke from a bushfire on the NSW Central Coast.

Smoke from a blaze that has destroyed more than 180ha near Lake Macquarie has been blown south by the northerly winds that have brought hot and dry conditions to much of the state.

The fire broke out near Catherine Hill Bay on Friday afternoon and is expected to double in size, says the Rural Fire Service (RFS).

"Because we have a northerly wind, that actually blew right down into the Sydney basin late last (Friday) night and has put up a huge amount of smoke," RFS spokeswoman Rebel Talbert told AAP.

"People can expect it (to stay) for Saturday and Sunday until we get that strong southerly change that we're expecting late Sunday night."

No homes are so far under threat but a number of spot fires have flared up behind containment lines and traffic in the area along the Pacific Highway may be affected, Ms Talbert said.

About 60 firefighters are battling the blaze, helped by a number of waterbombing aircraft.

"It's a large fire and it's going to take them a while to contain it," Ms Talbert said.

Health authorities have warned Sydney residents to take precautions as temperatures soar over the weekend into the mid-30s in Sydney and as high as the mid-40's in the city's west and across the state.

Children, the elderly and people with respiratory illness are especially vulnerable. They must keep cool and drink plenty of water, the RFS said.

A total fire ban is in place for the Sydney area and most of NSW except for the northeast area of the state.

Firefighter safety is an ongoing concern as they battle a number of fires in the heatwave conditions.

"A lot of those fires are burning in really inaccessible terrain and some of them have burning for about two weeks or more," Ms Talbert said.

"And we will be pulling crews out of those fires if it does start getting to be too dangerous, which is quite likely with the temperatures we're expecting."

Fire crews are also keeping a close watch on three fires in the Tumut area, about 90km west of Canberra, which, although contained, have blackened some 500ha of bushland.

Meanwhile, a fire that has destroyed more than 1100ha near Wyndham, in the southeast corner of NSW, is still burning out of control.

And more than 60 firefighters are battling a blaze in Wybong, northwest of Newcastle, that has burned out about 140ha, threatened homes and destroying a car.

"Our crews are on high alert - we've doubled our response to any new fire," Ms Talbert said.

"But on days like today firefighters are really put to the test. Anything over 40 degrees is very, very difficult for them."

Source: Associated Australian Press