Back-burning shrouds Sydney in smoke

A smokey haze has covered Sydney for the second time this month thanks to RFS back-burning.

A smokey haze has covered Sydney for the second time this month thanks to RFS back-burning. (AAP)

Sydney has been blanketed in another smoke haze as the Rural Fire Service carries out back-burning around the city.

The RFS is capitalising on the dry weather to finish hazard reduction burns around Sydney including in the Lane Cove National Park and Hawkesbury area on Thursday.

Back-burning will continue across the state in following days in areas including the Blue Mountains, the Central Coast and Hunter Valley, says the RFS.

"People can expect to see or smell smoke over the next few days," RFS Inspector Ben Shepherd told AAP.

The NSW EPA says the air quality is hazardous over Sydney's eastern and northwestern suburbs.

People with heart or lung disease, older adults and children should avoid exercising outdoors, according to a notice on the EPA's website.

The alert says all residents should reduce prolonged or heavy exertion and if anyone experiences breathing problems they should rest, use reliever medicine and seek medical advice if symptoms persist.