The Bureau of Meteorology says severe heatwave conditions are building across the state with temperatures in many regions forecast to soar past 40C.

NSW Health environmental health director Dr Richard Broome says Tuesday is likely to be "very hot and very smoky".

"Tomorrow is going to be a really grotty day," he told reporters on Monday afternoon.

"It's going to be putting a lot of stress on vulnerable people, particularly elderly people who have existing heat and lung conditions."

Sydney has suffered through some of the city's worst air pollution on record as bushfires have raged across the state in recent weeks.

Dr Broome said people with chronic health conditions needed to be very cautious and should take steps to reduce their exposure to heat and smoke.

"These conditions are likely to continue, so people need to manage their symptoms as best as they can."

Bureau weather services manager Jane Golding says temperatures on Tuesday could climb past 40C in Greater Sydney, the Hunter, the Illawarra and northern parts of the state.

"We've had a few days of respite from the smoke," she told reporters.

"But with the winds turning back around to northwesterlies tomorrow we are expecting it to be quite a smoky morning."

A southerly change should cool temperatures later in the week, Ms Golding said, although the wind could also fan flames on some fire grounds.

NSW Police deputy commissioner Gary Worboys reminded the public not to leave children or pets unattended in vehicles.

"Not only is it dangerous, it can be deadly," he said in a statement.

Australian Associated Press