A cyclone that left a devastating wake of extensive flooding and landslides in New Zealand has claimed at least four lives and police have "grave concerns" for other residents who remain unaccounted for.

Cyclone Gabrielle struck the country's north on Monday and has brought more destruction to this nation of 5 million than any weather event in decades.

Police said at least four people had been confirmed killed by the storm, including a child caught in rising water on Tuesday at Eskdale on Hawke's Bay. All four fatalities occurred near the same North Island east coast bay, two in landslides and two by drowning.

People await rescue from the rooftops of their homes in Napier. – AAP/NZDF

A weather station in the Hawke's Bay region recorded three times more rain over Monday night than usually falls for the entire month of February, authorities said.

Prime Minister Chris Hipkins said it was unclear how many people remained unaccounted for, with several communities still isolated by floodwaters, landslides and telecommunications outages.

Police said 1442 people had been reported uncontactable in the North Island as of Wednesday afternoon.

"We expect the vast majority of these people will be accounted for. However, there are several people missing for whom police do hold grave concerns," Hipkins told reporters.

Hipkins said he could not put a figure on how many missing person reports were of grave concern.

He said 1111 people had been reported found by Wednesday, which would cancel out some of those reported uncontactable. Around 9000 people have been forced from their homes since Monday.

A helicopter crew rescues a child from a rooftop in Napier. – AP/NZDF

More than 300 people were rescued Tuesday from the Hawke's Bay area, including 60 stranded on a single roof, an official said. The final 25 rescues of individuals and family groups were expected to be completed on Wednesday.

Hipkins said seven rescues had yet to be completed by late Wednesday. He did not say how many people had yet to be retrieved, but said none was in danger.

"The feedback that we've had is that those seven that are still outstanding, all of the people concerned are safe," Hipkins said.

"The people who were on rooftops and precarious positions have been rescued," he added.

Along with rescues, the government was prioritizing restoring power and telecommunications as well as delivering food, water and medicine to where it was needed, Hipkins said.A naval ship left Auckland late Wednesday with drinking water for Hawke's Bay communities and another ship would follow with vital supplies on Thursday.

A helicopter dropped bottled water for 3000 people on Wednesday night.

Emergency responders planned to hold a barbecue for 3000 people at the Hawke's Bay town of Wairoa.

Water flooding rural roads in Dargaville, in New Zealand's Northland region. – Reuters/E.Urlich

"They'll keep cooking into the night until either they run out of people to feed or they run out of food," Hipkins said.

Water treatment equipment would also be delivered to Wairoa on Thursday, he said.

Around 160,000 properties on the North Island were without power on Wednesday, down from 225,000 on Tuesday, the government said.

Princess Anne visited New Zealand's disaster management headquarters in the capital, Wellington, on Wednesday and praised the nation's response. Her visit to New Zealand was scheduled before the cyclone struck.

"My thoughts are with all New Zealanders whose homes or livelihoods have been affected by Cyclone Gabrielle," she said in a statement.

"I admire the courage of the people of Aotearoa during this alarming and difficult time.

Homes in the Esk Valley, near Napier, New Zealand, are flooded. – AP/NZDF

"You should all be proud of the resilience, strength and care for your communities you are showing in the face of adversity," she added.

As rivers broke banks and floodwaters rose, an estimated 400 people were rescued by boat, helicopter or Unimog trucks by the police or defence force.

Private helicopter operators joined the dash to winch people to safety.

"There was one very trying situation I was involved in where an elderly lady was stuck up a tree and her husband was on the roof," Rotorforce Helicopters chief pilot Joe Faram said.

"By the time we managed to extract them, I had to fly them directly to hospital, because she was quite hypothermic and very weak."

"I also had one case where I flew a man off a roof and he had his leg in plaster and a whole lot of young children and I flew a young couple off with a newborn."

A Category 2 cyclone through the tropics, Gabrielle carried the same intensity as it moved south to NZ waters, impacting North Island.

Water floods a street in Havelock North, southeast of Auckland. – AP

As of Wednesday, just two regions – Marlborough on South Island and the Wairarapa on North Island – are under heavy rain warnings, with the storm gradually petering out as it travels east into the Pacific.

Gabrielle has left a mighty toll, to lives, land and livelihoods.

"The severity and the damage that we are seeing has not been experienced in a generation,"  Hipkins said.

Whole towns have been cut off by the storm, with Wairoa in the northern Hawke's Bay and some Tairawhiti communities without power and unable to be reached by land, air or sea. Phone or internet connections are also down.

Emergency Management Minister Kieran McAnulty said establishing communication links and delivering the essentials was a priority with improved weather on Wednesday.

"I'm optimistic and hopeful but I'm also really worried," he said.

"I'm really worried about Hawke's Bay. There (are) still a few gaps in the knowledge that we won't know about for another couple of hours, in particular Wairoa," he said.

"They haven't had communications for a couple of days now.

"We know there's a shortage of food and we know there's a shortage of water so that will be the focus today."

The worst of the weather has now cleared and meteorological service WeatherWatch said in a statement that Cyclone Gabrielle is now east of the country and continuing to track away from the North Island with weather now improving.

Water gushes from a storm drain access port on a street in Te Awanga, southeast of Auckland. – AP

However, rivers in Hawke's Bay, a region on the eastern coast of the North Island, continue to pose risks and the local emergency management ordered further evacuations early on Wednesday. Electricity is out for around 225,000 customers across the island hampering clean up.

Mother of four Jennie Perris, who lives on a 10-acre (4-hectare) block of land on the outskirts of Whangarei, north of Auckland, said the family had been without power since Sunday.

Perris said the roads had cleared on Tuesday and the family been able to head into the city and shower at her mother's house, charge devices and stock up on bottled water but were now back to cooking on the barbecue.

"We're doing everything on it," she said.

Transmission companies around the country reported damage to substations and power networks.

About 225,000 people were left without electricity, while dozens of supermarkets closed, with Prime Minister Christ Hipkins urging New Zealanders not to panic-buy supplies.

People walk through flood water in Hastings, southeast of Auckland. – AP

Architect Lars von Minden, 50, lives in Muriwai, a beach town on the coast west of Auckland.

"I've seldom seen anything like it," he said.

"There are three or four areas where there are just these massive slips, some of them 300 metres (1000 feet) across, that have come down, taking out houses and roads and everything."

Authorities have evacuated beach settlements and are urging still more people to leave homes as rivers continue to swell and huge surf inundates beachfront properties.

Roads are closed, mobile phone services down and some towns cut off. Residents in hard-hit areas are being asked to conserve water and food because of fears of shortages. Air New Zealand restarted some flights in and out of Auckland, though many routes remained disrupted.

Helicopter and boat crews were rescuing people trapped by rapidly rising flood water in Hawke's Bay, southeast of Auckland.

A road between Napier and Wairoa is washed out by flood water. – AP/NZDF