The cyclone crossed the coast near the Maketu Peninsula on Sunday, packing destructive winds exceeding 130 km/h, heavy rain and large swells, national weather provider MetService said, describing Vaianu as a "life-threatening" system.
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 towards the fringes of the North Island, sparing Auckland from the worst conditions, said Emergency Management Minister Mark Mitchell.
But stronger winds and swells were still expected after its afternoon landfall.
"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," Mr 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.
"The concerning time is really from 2 p.m. this afternoon (0800 GMT) onwards when we've got high tides combined with those big swells," Mr Mitchell said.
The cyclone has forced hundreds of residents to evacuate and knocked out electricity to 5000 homes, with power restored to roughly 2000.
Authorities in the coastal Whakatane District reported a significant amount of damage as Vaianu passed through, with mandatory evacuations carried out at 270 properties.
New Zealand Defence Force members and heavy equipment have also been deployed to assist with evacuations.
MetService said it had recorded 130 km/h wind gusts in some areas, 24-hour rainfall totals of more than 100 mm in the city of Whangarei and wave heights exceeding six metres.
Air New Zealand said in a statement that it had cancelled more than 90 turboprop flights, primarily out of regional North Island airports.
"Domestic jet and international services are operating as scheduled, although there are some delays due to the weather conditions," the airline said.
Fire and Emergency New Zealand said it responded to more than 100 calls for assistance relating to wind damage and surface flooding.
Conditions will ease as Vaianu works its way down the North Island before exiting on Sunday evening at Hawke's Bay, MetService said.
"Things do improve from tonight and tomorrow, but at the moment this is still a life-threatening weather system," said Heather Keats, MetService head of weather news.
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.
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