A thick blanket of smoke has left Sydneysiders breathing "very poor" air, with more hazard burns expected to keep the haze around for another day.

Residents woke to the thick haze over the city on Monday as firefighters conducted backburning to prepare for a "significant fire season".

Westerly winds pushed the smoke over Sydney from hazard reduction burns carried out in the Southern Highlands on Sunday.

Smoke plume modelling indicated inversion, which locks air over low lying areas, would ease on Monday morning and the haze would dissipate soon after - but that didn't happen.

Air quality monitors reported high amounts of particles and low visibility in numerous suburbs - with "poor" and "very poor" air hanging around until late on Monday evening.

Hazard reduction burns continued through Monday as firefighters took advantage of still air and cool weather. That is expected to result in more smoke settling over the city until Tuesday afternoon, the spokesman said.

NSW Health's environmental health director, Ben Scalley, says small particles in the smoke could travel deep into people's lungs.

The elderly, children and people with heart and lung conditions were most at risk, Dr Scalley said.

"Smoke may aggravate existing heart and lung conditions and cause irritated eyes, coughing and wheezing.

"If their symptoms persist, it's important they do seek medical advice."

With time quickly running out before the summer fire season, the Rural Fire Service is planning a number of other burn-offs in the coming weeks to minimise risks after a dry winter has increased the fuel load throughout the state.

"We do have a lot of burns to do ... we have had below average rainfall in the majority of NSW and we are certainly cautious with no substantial rain forecast in the near future," another RFS spokesman said.

"They are predicting we are in for a very significant fire season ... it is vital we undertake the hazard reduction burns at this time of year when the conditions are right."

AAP