7.15am at the Sydney Opera House on Saturday morning.

7.15am at the Sydney Opera House on Saturday morning. Photo: Brianne Makin

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 blankets Sydney this morning, captured from the sky en route to Melbourne. Click for more photos

Sydney covered in fog

Fog blankets Sydney this morning, captured from the sky en route to Melbourne. Photo: Georgia Willesee

  • Fog blankets Sydney this morning, captured from the sky en route to Melbourne.
  • At 7.15am a tourist takes a photo of the Opera House as fog blankets Sydney.
  • Fog descends across Sydney and its suburbs - walkers in Queens Park in Bondi amidst the mist.
  • Morning fog on Sydney Harbour.
  • A surfer at Maroubra Beach looks out for obscured waves on the horizon.
  • Fog descends across Sydney and its suburbs-  exercise enthusiasts at Dover Heights amidst the mist during their morning training regime.
  • Sydney Opera House is disguised by the fog early this morning.
  • A cyclist in Maroubra negotiates the mist.

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.