At least one flight has been diverted, with the aircraft - which was scheduled to land at 9.30am before returning to Sydney at 10am - making an attempt to land before heading to Bathurst instead.

A second flight from Melbourne, which will then be heading on to Brisbane, was expected to land just before 12 noon, but has also been diverted to Bathurst.

Passengers for the Regional Express Sydney flight were told they will be bussed to Bathurst at 11.45am, looking to board a plane to the state's capital at about 1pm.

One traveller at the airport said the fog was "reasonably thick" but was "coming and going", with moments looking clear, only for clouds to return minutes later.

A spokesperson from the airport could not be reached for comment.