Fog and low cloud delayed flights from Melbourne Airport as visibility closed in to just 50 metres.
Heavy fog blanketed Melbourne from 3am on Wednesday with some flights delayed by more than half-an-hour while others were diverted.
"We observed visibility at Melbourne Airport down to 50 metres so roughly if you were kicking a goal in the footy you wouldn't be able to see the goal posts from the 50 metre line," senior forecaster Tom Delamotte said.
Low cloud and hazy conditions persisted until early afternoon.
Live flight monitoring sites showed average delays of 34 minutes for flights departing Melbourne at lunchtime, with 68 per cent of flights delayed.
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However, earlier data showed passengers were delayed leaving Melbourne by almost an hour with 74 per cent of flights impacted by the heavy fog at 10am.
A Melbourne Airport spokeswoman said not all flights were affected.
"Some international planes can usually navigate through fog, it all depends on the type of equipment the plane is equipped with," an Airservices Australia spokeswoman told AAP.
Passengers were told to check with airlines for more information.
Easterly winds from Gippsland pushed moist air into Melbourne, causing the heavy fog when it hit colder temperatures, the Bureau of Meteorology said.
"Fog in August isn't unusual we usually see one or two events ... but probably the extent of the fog is what's little bit more unusual for this time of the year," Mr Delamotte said.
VicRoads warned drivers to take care on roads, slow down and use their low-beam lights or fog lights in areas of poor visibility.
