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.
Fire behaviour analysis undertaken by the RFS on behalf of National Parks and Wildlife warned of smoky conditions across Sydney, the worst of which is expected on Saturday.
RFS smoke prediction modelling suggested smoke from the Hawkesbury burn would be pushed across Sydney by westerly winds during Saturday morning and out to the ocean by lunchtime, before a change in wind direction would blow it back through the city and to the north-west.
The RFS recommends those with breathing issues or other conditions impacted by air quality to reduce outdoor physical activity.
Under the air quality index,"Very Good" registers from 0 to 33 for all categories, "Good" is from 34 to 66. "Fair" is between 67 and 99, leading to "Poor" (100-147), "Very Poor"(150-199) and "Hazardous" (200 and above).