Heavy fog has forced international flights to land in Adelaide and is delaying domestic flights from Melbourne Airport.

On Nov. 14, Melbourne Airport spokesman Tom Perry said the morning's thick cloud had forced three international flights to land in Adelaide and delayed arrivals by more than three hours.

Mr Perry said the fog had lifted at Tullamarine but he expected departure delays to last well into the afternoon.

"There are certainly queues but we are not stopping planes from departing," he said.

Three international flights, Qantas flight 180 from Tokyo, Malaysia Airlines flight 149 and Cathy Pacific flight 135 from Hong Kong, were diverted to Adealaide but the Qantas and Malaysia Airlines flights had since landed safely, he said.

The Cathy Pacific flight was expected to land shortly.

Mr Perry said international aircraft were being turned away because the large aircraft had insufficient fuel to circle Melbourne Airport.

"If they (flight control) are concerned they have to circle the airport they will make the decision as to whether they should consider the risk," he said.

Mr Perry said domestic passengers flying out of Melbourne in the afternoon should check with their airlines or log on to www.melbourneairport.com.au.

A Bureau of Meterology spokeswoman said she expected the fog to clear in the next couple of hours and the low clouds were unusual for this time of year.

"Usually the sun is strong enough to dissipate the clouds," she said.