Flights from Hamilton and Gisborne are among those affected.
There were long queues at Auckland Airport, but a spokeswoman said these were moving steadily.
Weather Watch analyst Philip Duncan said there were big fog patches in Waikato, Auckland, the central North Island and even the lower part of Wairarapa.
He said because there was no wind, the fog might stick around for quite some time in some areas.
Philip Duncan said there were very still conditions across the North Island.
He said the fog might linger right up to lunchtime and in some areas, just after lunchtime.
Fog was again possible on Sunday morning.
A reporter in Mt Albert said the fog was thickening about 9.30am. Visibility was about 100m at best and the blue skies had disappeared, she said.
Passengers should check Auckland Airport's website or its app for the latest flight arrival and departure information.