Sydney smoke to linger three more days

Smoke shrouding Sydney is expected to remain in place for three more days before dissipating

Heavy smoke will linger in the Sydney basin for at least three more days, with authorities advising those most at risk to stay indoors.

The harbour city has on Tuesday been ringed by large bushfires in the Blue Mountains, near Warragamba Dam and in the Hawkesbury area.

A "temperature inversion" in which warm air overlays cooler air in the atmosphere, meanwhile, has trapped the smoke close to the ground and prevented its dissipation.

Eastern and southwest Sydney were deemed "hazardous" for air quality, as was the Upper Hunter and parts of the Lower Hunter.

Northwest Sydney was deemed of "good" quality.

The lingering smoke has prompted a NSW Health warning for those with heart and lung problems to stay indoors and avoid exercise.

"The best thing you can do is stay indoors, keep your doors and windows shut and if you've got air conditioning, turn that on as well, which helps to clean the air inside," NSW Health's Dr Richard Broome said in a statement.

Face masks are considered ineffective as they fail to filter out smoke particles.

The Bureau of Meteorology's Shuang Wang said the Sydney temperature inversion was likely to persist until at least Friday.

She said westerly winds were blowing smoke right into Sydney.

"It depends if the fires come under control ... a southeasterly change late on Friday (and) conditions would improve," Ms Wang told AAP.

Two million hectares of NSW land have been burnt since July in more than 7000 fires, with authorities dubbing it the "most challenging bushfire season ever".

Six people have died while at least 673 homes have been destroyed.