Winds up to 130km/h have downed power lines and briefly closed the Auckland Harbour Bridge.
Experts warn of more power cuts and fallen trees, urging caution and preparedness.
Weather experts say it is not unusual for a cyclone to be making waves at this time of year but warn ex-tropical Cyclone Tam is on the upper end of the scale.
Strong winds and heavy rain began lashing the upper North Island on Wednesday, with gusts of 130km/h being recorded in Cape Rēinga, while winds also brought down power lines and the brief closure of the Auckland Harbour Bridge.
Maritime NZ said some of the winds forecast for the top half of the North Island are the strongest since 2017.
While the winds are expected to be more intense, Niwa meteorologist Seth Carrier believed certain areas will be more affected.
“I would not expect things on the scale of Cyclone Gabrielle. This event is much weaker.
“Beaches that are facing the east will be exposed to the strongest winds. We have already seen power cuts, and I wouldn’t be surprised to see more because we haven’t seen the strongest winds yet.
“There will be impacts, but I don’t think we are on the same scale as Gabrielle.”
Professor James Renwick, from Victoria University in Wellington, said New Zealand’s location meant it was always likely to be in the firing line, especially at the tail end of the cyclone season.
Renwick said the winds are likely because of a combination of the low pressure of Cyclone Tam and the high pressure south of New Zealand.
“We have a big difference in pressure but it’s a consequence of the intensity coming out of the tropics but [also] the intensity of the anti-cyclone down south,” said Renwick.
“It’s that pressure difference that gives you strong winds. Storms this intense are rare and it is at the upper end of the scale.”
Associate professor Daniel Kingston at the University of Otago said climate change has also played a role in creating more intense storms.
“If we have sea-surface temperatures warming, it’s warm sea conditions that can provide fuel to cyclones to an extent.
“We have a warming atmosphere, which can hold more water as it warms, so we’ve experienced that 1.1C increase over the last century.
A map showing ex-tropical Cyclone Tam off New Zealand at 3pm on April 16. Photo / Windy.tv
“That allows that bit more moisture in the atmosphere, which intensifies these systems.”
Renwick echoes Kingston’s comments.
“It’s what the Pentagon and military call a threat multiplier,” Renwick said.
“It makes everything more intense because we are adding energy to the climate system when you have a storm. Climate change will be making things more intense.”
With the winds intensifying, there is the chance of more fallen trees and power lines, which can put people in danger.
Massey University emergency management lecturer Dr Lauren Vinnell said people need to trust their gut when making decisions but also be wary of their surroundings.
“It’s important not to make any quick decision, but if you feel unsafe, act immediately,” she said.
“It is important to assess and think if you’d be putting yourself in more danger. It’s escalated quickly, so some people might feel unprepared. In most cases, trusting your gut is the right thing to do.
“High winds can start fires and bring down power lines, so people will need to be conscious of what is around them.”
The upper North Island will remain under heavy rain and strong wind warnings during Thursday, with impacts possibly reaching as far as the Tasman region over the weekend.
