At least one cyclone-related death has 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 flooding forcing 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 including 1000 in Northland, 400 in Auckland, 93 in Bay of Plenty and 75 in the Waikato.
In the eastern Hawke's Bay and Tairawhiti regions, that number is more than 1000, with a full assessment impossible due to blocked communication lines.
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 told Radio NZ.
"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 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.
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," 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. Phone or internet connections are also down.
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.
"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."
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.
The confirmed death occurred in the northern Hawke's Bay town of Putorino, where a woman was killed when a landslip collapsed the house she was in.
A body was found on the beach in Bay View, a hard-hit area north of Napier, with police investigating.
There are also fears for a volunteer firefighter trapped in another collapsed home, in Muriwai, west of Auckland.
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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