Two fires on on the fringes of Sydney has reduced air quality in the city, and spreading a strong smell of smoke.

Fire behaviour analysis undertaken by the Rural Fire Service on behalf of National Parks and Wildlife warned of smoky conditions across Sydney, the worst of which is expected on Saturday

Prediction modelling suggested smoke would be pushed across Sydney by westerly winds during Saturday morning and out to the ocean by lunchtime.

The RFS recommends those with breathing issues or other conditions impacted by air quality to reduce outdoor physical activity.