All flights into Auckland Airport are being diverted because of debris on the main runway caused by strong winds.
The airport said flights were either being returned to their originating airport or Christchurch.
There is no estimated time for when the runway will reopen, it said.
MetService said the winds have eased but would remain strong overnight in Auckland, Coromandel and Waikato.
Civil Defence said it was not activating or mobilising but were standing by to monitor the response.
Power is out to many parts of New Zealand's biggest city after sustained winds of 100km/h winds battered the area.
MetService said a 133km/h gust of wind has been recorded at Auckland Harbour bridge.
Niwa said the wind gusts around Auckland this evening were the same strength as a category one cyclone.
Mangere had gusts of more than 100km/h for three consecutive hours - between 8pm and 11pm.
Niwa said winds of this magnitude were typical of category one cyclone intensity.
Power company Vector said there were widespread outages affecting the central city and many western suburbs, with about 15,000 people without power.
In some areas where power is out, there are no street lights and some traffic lights are also out.