At least two cyclone-related deaths have been confirmed and there are fears the toll will rise as links are re-established with remote towns cut off by the storm.
The Hawke's Bay region appears hardest hit, with about 9000 displaced as burst rivers and flooding forced evacuations in a string of towns and suburbs.
"It's almost beyond belief," Emergency Management Minister Kieran McAnulty told Radio NZ on Wednesday.
The NZ government says about 2500 people are displaced and under evacuation orders elsewhere, including 1000 in Northland and 400 in Auckland.
In the eastern regions of Hawke's Bay and Tairawhiti, a full assessment is impossible due to blocked communication lines and access.
Defence Minister Andrew Little said 200 people and five dogs had been rescued of roofs in rising floodwaters by defence personnel in the Hawke's Bay.
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."
A Category 2 cyclone through the tropics, Gabrielle carried the same intensity as it moved south to NZ 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, to lives, land and livelihoods.
"The severity and the damage that we are seeing has not been experienced in a generation," Prime Minister Chris 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 on Tuesday.
Phone and internet connections were also down, but gradually being restored.
Mr 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 told Newstalk ZB.
"We know there's a shortage of food and we know there's a shortage of water (in Wairoa) so that will be the focus today."
More than 225,000 people were without power on Tuesday as electricity transmission agency Transpower warned of a "grid emergency".
Mr McAnulty said that number had eased to 145,000 by Wednesday morning, with much of Gisborne back online.
Also on Wednesday, authorities confirmed two deaths.
A body was found in a collapsed house where Fire and Emergency NZ had been searching for a missing volunteer firefighter since Monday night.
In the northern Hawke's Bay town of Putorino, a woman was killed when a landslip collapsed the house she was in.
A third death may be linked to the cyclone, with police investigating the circumstances of a body found on the beach in Bay View, a hard-hit area north of Napier.
While power and communications are being restored as the weather improves, many major highways have suffered extensive damage due to flooding and landslips.
NZ 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.
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