Sydneysiders were greeted by the strong smell of smoke on Friday morning, believed to be from two fires on the city's outer fringes.

The state government's air quality monitoring systems for the Sydney region tipped from "Good" to "Fair" between 6 and 7am as the NEPH reading for visibility, which measures fine particles in the air, reached 77.

Smoke from a bushfire in Campbelltown - under control since Thursday afternoon - is believed to be partly responsible, although the Rural Fire Service said on Friday morning that the 118-hectare blaze was "contained and the majority of that fire is out".

The National Parks and Wildlife Service NSW began a planned three-day, 900-hectare backburn in the Hawkesbury on Thursday, but cut it short to just 100 hectares. It is possible residual smoke from the Blaxland Ridge hazard reduction activity contributed to the smoke in the city.

The smoke is expected to lift as the morning warms up, dropping to "Good" at the next reading, at 65.