Up to 15,000 homes are without power as a fierce storm with the force of a Category 2 cyclone in places pummelled Auckland overnight with gusts of up to 140km/h.
Flights were grounded at Auckland Airport, homes plunged into darkness and there were reports of building swaying and glass flexing in the high winds.
Trees toppled onto homes and blocked roads, trampolines flew into other people's properties and Fire and Emergency personnel were overwelmed by callouts.
Train and ferry services were halted and Niwa said the storm has the same force as a Category 2 cyclone.
A tree fell on a car in New North Rd in Morningside, trapping a woman. She was taken to hospital with moderate injuries.
There are other reports of trees down on cars and homes, and the roof was ripped from a large structure in Hobsonville Point.
Jason Robinson said he just got home in Hobsonville from work when he heard a noise outside so he checked to see if his vehicle had been damaged.
When he inspected the car he heard a "loud groaning noise" and turned around to see a building about four storeys high slowly being blown over.
"It was like a slow creaking sound."
The building was a base for a building firm that constructed apartments and homes.
"It is a size of a small warehouse and it came crashing down."
"It's like a twisted wreck."
Fire and Emergency New Zealand received dozens of calls about trees toppling onto roads, power lines and homes in the Auckland region.
Its Northern Communication Centre was inundated with 111 calls because of the extremely strong winds.
They dealt with more than 170 calls in Auckland and the wider Upper North Island area.
"Most of the calls are about trees that have fallen down, or roofing coming off buildings. A tree has fallen through the roof of a house in Stanley Pt, and another has fallen through the roof of a home in Birkdale.
Two trees have also toppled onto a house in Massey. A tree took out power lines on West Coast Rd in Waiatarua. Flooding has been reported in parts of Coromandel.
MetService meteorologist Ravi Kandula reported gusts of up to 140km/h around Manukau Heads.
Auckland's Airport had winds of 90km/h and gusts reaching up to 120km/h.
Herald readers have described fierce gusts making windows flex and lights flicker.
Dramatic lightning strikes also brought the dark sky to brilliant life.
Gary Singh told the Herald his family of five had been in the dark with a torch after losing power in their Papatoetoe home. They sat on a bed listening to the wild weather outside.
"It's quite scary. The winds are quite strong."
He had tried ringing Vector but was told the company was only taking emergency calls because of the volume of calls.
An Auckland Airport spokeswoman told said the main runway had been closed because of debris and all flights diverted.
A Mt Roskill resident said his house was damaged when his neighbour's trampoline flew into his property.
"It sounded like a cannon firing then it flew around and finally lodged itself on the driveway. Crazy evening with lightning, power surges and bang! No one is hurt. Stay safe."
Kingsland resident, Stuart Sonman-Lund said the power was out in his house, where sirens could be heard in the distance.
The winds were hitting the house so hard it felt like an earthquake, he said.
Other parts of the country are feeling the drop in temperatures as snow is forecast down to 700m in Gisborne and Hawke's Bay.
"That will be easing gradually from tomorrow afternoon."
The South Island is now clear of warnings but rain will continue north of Banks Peninsula and Kaikoura.
Winds were also very strong in the Marlborough Sounds and in Wellington.
"They will continue to buffet through the day and ease from tomorrow."
A widespread drop in temperatures is accompanying the storm, which is sweeping up the country, leaving a trail of destruction.
A twister swept through National Park this morning, where it ripped off roofs, downed trees and demolished a house.
Power has now been restored, after wild weather cut the region off the grid.
However, police and fire crews remained on the ground in the central North Island assisting those affected by heavy rain and winds.
The MetService has recorded nearly 250 lightning strikes in Auckland and Northland in the past two hours.
All ferry journeys in the Cook Strait have been cancelled until 3.30am on Wednesday, according to the Interislander's website.
Chilly front moving up the country
According to forecaster WeatherWatch, a deepening low sitting over the North Island has been holding a southerly change from spreading any further northwards.
However, the top of the Island is expected to experience a significant drop in temperatures overnight.
Temperature highs in spots such as Hamilton, Auckland, Tauranga, Whangarei and Kaitaia are mostly around 13C, with 15C tomorrow - about 10C to 12C lower than they have been in recent weeks.
Temperatures in Napier and northern Hawke's Bay will also plummet, falling to between 15C and 18C colder than they were a week ago.
While things will take a turn for the chillier up north, the South Island is more likely to see a small increase in temperatures overnight.
A high pressure system is expected to begin to build behind a southerly from the Arctic, which is slowly moving north and east.
The combination of these two changes would likely result in places like Gore, Dunedin and Invercargill going up two or three degrees on Wednesday.
Trail of destruction left by tornado
Eight houses were damaged by this morning's tornado in Rahotu, Taranaki, three of them severely.
People needing get help can go to Rahotu School.
About 12,000 houses are without power across South Taranaki.
Taranaki Civil Defence this afternoon was warning of wind gusts of up to 120km/hour between 3pm and 11pm tonight.
Ripped-off roofs and downed trees and powerlines left by the storm have served another dramatic reminder that tornadoes can strike New Zealand - and hard.
But they're still very uncommon: on average, about seven to 10 moderate to strong tornado events are reported each year.
Because they're extremely localised, the damage they cause is very confined to the tornado itself, although the violent winds can fling debris hundreds of metres.
Roads closed, speed limits in place
Speed restrictions are in place on Auckland's Harbour Bridge, due to "significant" wind gusts.
Ferries are also cancelled from Northcote Pt wharf, which is closed until further notice.
Buses were operating between Birkenhead and Northcote Pt from 4.20pm to 6.50pm.
Police and fire crews are on the ground in the central North Island and on the West Coast of the South Island assisting those affected by heavy rain and winds.
A number of roads are affected by surface flooding and trees and power lines have been brought down.
SH6 is closed between the townships of Ross and Franz Josef Glacier after trees were brought down at Pukekura.
Police said they are not aware of any weather-related injuries.
Motorists were being asked to follow the directions of emergency services and roading contractors around local road closures.
Non-essential travel should be delayed and motorists are being asked to take extra care.