Hawke's Bay is emerging as the hardest-hit region, where 9000 people have been displaced, and a child lost in floodwaters is among the dead.
There are fears the death toll will rise as flooding subsides and links are re-established with remote towns cut off by the storm, New Zealand's worst in 35 years.
"It's almost beyond belief," Emergency Management Minister Kieran McAnulty told Radio NZ.
Police say they have had 1442 reports of uncontactable people, most from the Hawke's Bay and Tairawhiti regions in the North Island's east.
While police expect "the vast majority" of those to be the result of downed communication networks, police hold grave concerns for "several people".
Prime Minister Chris Hipkins said there was "significant concern" for a number of people missing in Hawke's Bay.
"Today we are getting a better picture of the damage across the North Island," he said.
"What is clear is that the devastation is widespread and it's taken a toll beyond property and livelihoods, to people."
Mr Hipkins said the focus on Wednesday was rescue missions, restoring power and telecommunications, conducting aerial surveys and deploying emergency supplies.
Defence Minister Andrew Little said 200 people and five dogs had been rescued from roofs in rising floodwaters by defence personnel in Hawke's Bay, where police and 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 told Radio NZ.
"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 new-born."
Three of the deaths have occurred in Hawke's Bay.
A child's body was recovered in Eskdale, another body was found on the beach in nearby Bay View, and a house collapsed on a woman in Putorino under a landslip caused by heavy rainfall.
A volunteer firefighter's death was also confirmed in Muriwai, on Auckland's west coast, on Wednesday.
"The grief of their families and loved ones will be unimaginable," Mr Hipkins said.
A category-two cyclone through the tropics, Gabrielle carried the same intensity as it moved south to New Zealand waters, impacting North Island from Sunday to Tuesday.
It is gradually petering out as it travels east into the Pacific, but has left a mighty toll.
Whole towns were 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.
Communications were re-established on Wednesday, with urgent missions underway to supply food and water.
The defence force has been scrambled to assist, with NZDF 700 personnel, four aircraft, seven helicopters, two ships and 58 trucks deployed across the country.
HMNZS Manawanui has sailed from Auckland bound for Gisborne, dropping off food and water into Tairawhiti along the way.
Around 160,000 households remain without power on North Island, down from 225,000 on Tuesday.
While power and communications are being restored as the weather improves, many major highways have suffered extensive damage due to flooding and landslips.
New Zealand is in a national state of emergency for at least seven days, with Mr McAnulty anticipating a "long and slow" recovery.
"This is not going to be a quick fix and it's not going to be cheap but we're committed to do it," he said.
