A heavy fog descending on Melbourne on Wednesday evening could hang around for days, according to the weather bureau.


Senior forecaster Richard Carlyon said perfect conditions of high humidity and light wind meant the fog had crept north on a sea breeze from Port Phillip Bay.


By 6pm it had settled in over Melbourne's CBD but was expected to expand further north overnight, and recur for the next three mornings.



"It looks likely to be quite thick fog and visibility is likely to drop to below 100 metres during tonight (Wednesday) and tomorrow morning," Mr Carlyon said.