Several international flights have been diverted to other airports and other flights have been delayed as thick fog descended on Sydney this morning.
Singapore Airlines' SQ221 was diverted to Melbourne, while Qantas's QF12 and United Airlines's UA839 and UA863 were diverted to Brisbane, Sydney Airport said.
"Full operations will resume as soon as the fog lifts but there will be consequential delays as flight schedules progressively return to normal," a Sydney Airport spokesman said in a statement.
The fog formed in the western parts of Sydney before midnight and expanded east, appearing in the city and at the airport from 4am, Bureau of Meteorology forecaster Sarah Hicks said.
"It's been reducing visibility at the airport down to about 600 metres but I think on occasion it might have been fluctuating a little bit around the area," she said.
"In the western part of the Sydney area, it's even more reduced. Richmond's about 250 metres visibility and Camden is reporting 50 to 100 metres visibility."
The thick fog - at a depth of about 800 feet - was quite widespread throughout the Sydney Basin, through the Hunter Valley, out towards Dubbo and was also affecting other parts of the state near the Victorian border, Ms Hicks said.
The fog was expected to lift about 10am to 11am in Sydney, she added.
"What's causing the fog is we had fairly moist conditions due to the rainfall that we had yesterday and overnight because it was a very clear night, the air was able to cool quite a lot ... down to the point where the water vapour became liquid, forming a 'cloud' on the ground," Ms Hicks said.
The fog conditions were not expected to return to such levels over the next few days, she added.
Latest flight changes will be updated on Sydney Airport's website.