Hazardous air quality alerts were issued for multiple locations across the state as the population awoke to the choking smoke.

Air quality in Sydney was considered as bad as ever, while in Newcastle visibility was restricted to hundreds of metres.

Fine particle pollution was worse in the Upper Hunter, reaching hazardous levels at both Singleton and Muswellbrook.

NSW Health advised people to stay indoors, with windows and doors closed, or to stay in air-conditioned premises.

Groups sensitive to air pollution were advised to completely avoid all outdoor physical activities, while it was suggested everyone "significantly cut back".

"We are urging people to take these conditions seriously," director of environmental health Dr Richard Broome said at a media conference in Sydney on Tuesday.

"We are anticipating that the smoke will continue right through to tomorrow."

Dr Broome said the air quality was among the worst seen since the 2009 dust storm.

"This smoky period that we've been experiencing for the past month or so is certainly unprecedented," he said.

"These conditions are a risk to people's health. Most people are going to be experiencing sore eyes, nose or throat, or perhaps a big of a cough, but there are some other people who will be finding its making there symptoms worse.

"The people at greatest risk are those with existing heart and lung conditions ... young children are also at more risk because they have developing lungs and are more inclined to exercise vigorously outdoors. The elderly as well because they're more likely to have underlying conditions."

NSW Ambulance superintendent Brent Armitage said paramedics would typically respond to about 70-100 instances of respiratory illness on a day like Tuesday. "These range from minor respiratory conditions right through to critical asthma," he said.

Thousands of construction workers downed tools in Sydney while many schools in the Hunter kept students inside during normal outdoor activity periods.

Schools at Congewai, Millfield and Broke were closed due to nearby fires.

The Bureau of Meteorology had issued a fire weather warning for the Greater Hunter, which was also slated with with a severe fire danger rating and a total fire ban.

Temperatures exceeded 40 degress across much of the region, except for coastal areas.

A regional high of 42 degrees was recorded at both Tocal and Scone.

Maitland and Merriwa hit 41, while Singleton, Cessnock and Cooranbong all hit 40.

On the coast, a high of 33 degrees was recorded at Nobbys and 27 at Norah Head.

The fire danger rating for the Greater Hunter was listed as high for Wednesday.

Fires continue to burn at advice level in the Hunter at Pelaw Main, Paterson, Wollombi and near Barrington Tops.

Larger blazes are burning further south in the Wollemi National Park and to the north across the Mid North Coast.