Hazard reduction burns are underway in parts of NSW, shrouding parts of Sydney in a smoke haze.

Some 39 fires have been lit to reduce fuel loads on thousands of hectares across the state, including in the Blue Mountains and the South Coast which were hard-hit by catastrophic bushfires in 2019-20.

The hazard reduction burns, expected to continue for several days, prompted the NSW Rural Fire Service to issue a smoke advisory for greater Sydney, the Southern Highlands and the Illawarra region.

Air quality on Tuesday morning dropped to “extremely poor” in Campbelltown, south-west of Sydney, “very poor” in Bringelly, Camden, Liverpool and Chullora, and “poor” in Lidcombe, Rozelle, and Kembla Grange in Wollongong.

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NSW Health warned anyone with heart and lung conditions to limit outdoor activity when air quality was poor.

“Smoke from hazard reduction burns can be patchy and conditions often change rapidly,” NSW Health’s Dr Richard Broome said.

“Over the coming days, the smoke is expected to be worse in the mornings, so people are advised to limit outdoor activity until it clears in the afternoon.”

By early afternoon, all locations across the state had returned to “fair” or “good” air-quality readings.