The Transport Agency is urging drivers to take care, with a blanket of fog covering parts of Auckland.
It says visibility is reduced on some motorways and is reminding drivers to increase following distances.
- Weather: Half of school holidays to be wet, windy, miserable
- Weather to put a damper on school holiday traffic
Video posted to Twitter shows cars' lights shrouded by a thick blanket of fog.
Fifteen domestic flights out of Auckland Airport have been cancelled and a further 26 delayed.
International flights have not been affected.
Video from NIWA shows the fog rolling in across the city just before 6am, with skyscrapers and buildings disappearing under a thick blanket of cloud.
It's a disappointing start to the school holidays, which are nearly halfway through their first week.
The South Island had a particularly rough start to the break, with Southland lashed by strong cold winds over the weekend.
It didn't get better when the week began with warnings Southland and Otago could be affected by snow flurries, and temperatures dropped a few degrees at the bottom of the North Island on Monday.
Things are expected to settle down towards the end of the week, but weather models are indicating a wet weekend ahead.