Sydney has woken under a thick blanket of fog, with visibility in parts of the west at less than 50m.
The fog is widespread over Sydney, causing delays for flights coming in to Sydney Airport and suspending all ferries.
Camden airport is reporting visiblity at less than 50m, and fog is similarly thick around Sydney's west and south-west where visibility is as low as 100m.
Weatherzone meteorologist Guy Dixon said "high relative humidity and calm winds" are likely causes for the fog, but conditions should clear into the mid-morning.
Mr Dixon said Sydney Airport is recording visibility down to 400m, which is reportedly impacting some flights.
Fog is causing some diversions this morning - please check with your airline for updates on your service— Sydney Airport (@SydneyAirport) August 8, 2014
Heavy fog is affecting all services using Sydney Harbour. Passengers are advised to allow extra travel time and have a back up travel plan.— 131500 ferries (@131500ferries) August 8, 2014
Qantas has said five international flights have been diverted due to the fog, heading to Adelaide, Brisbane and Melbourne where they will refuel.
Two domestic Qantas flights have also been diverted.
All ferries, including Manly ferries have been suspended until further notice.
Transport management spokesperson Leighton Archer said there is no forecast as to when services will resume.
"It all depends on when the fog lifts. Passengers are advised to delay their journey, use alternative forms of transport and allow additional travel time," he said.
Zach Berry-Porter at the Bureau of Meteorology said the foggy conditions are a combination of very light winds overnight and a high pressure system dominating NSW.
"Once the fog clears conditions are mostly sunny with top of 21," he said.
Conditions for the Sun-Herald City2Surf on Sunday will be fine, with a top of 20 for the 14km dash. Starting conditions are likely to be similar to last year's run, with an overnight low of 9 degrees and the chance of a morning shower.
However Mr Berry-Porter said there will be a cool change late in the day. The change should lead the way to partly cloudy days and aa few showers with a temperature range of 8-17 degrees until Thursday before temperatures warm up slightly, according to the Bureau of Meteorology.