Flight disruptions are affecting Christchurch, Hamilton, Auckland, Taupō, Rotorua, Tauranga, a Hamilton Airport spokesperson said.
A passenger on a flight which left Hamilton for Christchurch on Tuesday night at 6.15pm said the pilot announced at 7.30pm that fog had set in earlier than expected in Christchurch, and the plane would have to be diverted back to Hamilton.
The Hamilton Airport spokesperson said aircraft were on the ground and ready for an improvement in the weather.
Air New Zealand chief operating officer Alex Marren said fog-related disruptions on Tuesday had continued into Wednesday.
"Due to lengthy periods of fog across many parts of New Zealand on yesterday [Tuesday], a number of flights across the country have been disrupted, flowing into today.
"Our teams are monitoring the conditions closely and working hard to get people to their destinations, repositioning both aircraft and crew. Customers booked to travel are advised to keep an eye on the Air NZ app or our website for up to date travel information."
Marren thanked Air NZ customers for their understanding and patience.
Christchurch Airport confirmed fog had cancelled several inbound and outbound flights so far this morning and there was no indication of it lifting.
You can check the status of flights into and out of Christchurch Airport here.
Wellington Airport said weather conditions in the capital were not affecting flights, however, the impact at other ports was causing flow-on cancellations and delays.
Jetstar said it had not had any fog-related cancellations on Wednesday, but two "minor delays" to flights out of Christchurch.
On Tuesday, thick fog also affected Upper North Island airports, including 20 domestic flight cancellations at Auckland Airport alone.
Auckland Airport has been contacted for comment.
