Cyclone Vaianu brought destructive winds exceeding 130 kph, heavy rain and large swells even before it was expected to make landfall in the afternoon, according to national weather provider MetService.
Authorities have placed several regions under emergency declarations and issued "red" level wind warnings, which are reserved for only the most extreme weather events.
The cyclone was tracking "more towards the fringes" of the North Island, sparing Auckland - the country's most populous city - from the worst conditions, Emergency Management Minister Mark Mitchell said. But stronger winds and swells were still expected when it crossed the coast.
"It's moved more to the fringes and more to the east, which means that we haven't quite seen the intensity that we had prepared for or that we thought we were going to get hit with. So that is good news," Mitchell said.
"In terms of what we're going to expect over the next 12 hours is we are going to see a bit more of intensification, as the cyclone starts to come down and get closer."
The combination of the afternoon's high tide and large swells from the storm could trigger coastal inundation, Mitchell warned.
"The concerning time is really from 2pm this afternoon onwards when we've got high tides combined with those big swells," he said.
The cyclone has forced hundreds of residents to evacuate and knocked out electricity to some 5,000 homes so far, with power restored to roughly 2,000, he said.
New Zealand Defence Force members and heavy equipment have also been deployed to assist with evacuations.
MetService said it had recorded wind gusts of over 130 kph in some areas and 24-hour rainfall totals of over 100 mm in the city of Whangarei.
Vaianu has conjured up memories of 2023's Cyclone Gabrielle, which killed 11 and displaced thousands in one of New Zealand's biggest natural disasters this century.
