Fog blanketing Wellington Airport and disrupting almost all flights may not clear until mid-morning.

The fog rolled into the city yesterday afternoon, causing the cancellation of almost all arrivals and departures.

Most arrivals and departures through yesterday afternoon and evening were grounded.

There have been more delays and cancellations of both local and international flights this morning.

Wellington Airport says the airport is open and operating as usual and it's up to the airlines to decide if they'll fly.

A meteorologist with the forecaster MetService, Kyle Lee says a mass of humid air tracked north over the country, formed sea fog over the cooler water of Cook Strait, and was then funnelled into the harbour.

He expects it will clear by mid-morning.

Yesterday Air New Zealand said its smaller, turboprop regional flights were the only ones being affected, but further cancellations included larger planes travelling to the main centres, and to Australia.

Several of the regional turboprop flights were forced to return to their point of departure.

-RNZ.