Emergency Management Minister Kieran McAnulty made a declaration at 8.43am on Tuesday as Kiwis woke to flooding, power loss, road closures and phone network outages from the massive storm.
Thousands of people have been evacuated across the country and fears are held for the lives of several New Zealanders.
In Muriwai, a coastal town west of Auckland, a volunteer firefighter is missing feared dead, and another is in a critical condition after a home collapsed under a landslide.
The navy is also searching for two boats in waters north of Auckland.
News outlet Stuff also reports people are trapped in cars along State Highway 5 between Napier and Taupo, where communications are patchy.
"This is an unprecedented weather event that is having major impacts across much of the North Island," Mr McAnulty said.
"This is a significant disaster with a real threat to the lives of New Zealanders."
It is the third time a national declaration has been issued, after the 2011 Christchurch earthquakes and the COVID-19 pandemic, showing the scale of the damage.
Overnight, Gabrielle continued its southwards march before parking near Great Barrier Island, to the north of the Coromandel Peninsula.
It is expected to move eastwards, and by 10am it was positioned in the Bay of Plenty, north of Tauranga.
Its massive size is whipping up destructive winds all across North Island.
More than 100,000 Kiwis have lost power in the past 24 hours, including the entire city of Napier, which lost power on Tuesday morning.
Regional states of emergency have been declared in at least 10 regions of North Island, including Auckland, Northland, Thames-Coromandel, and at 4.30am on Tuesday, Napier and Hastings.
MetService executive Lisa Murray said some areas of the Coromandel, the region closest to the storm's eye, received 300 millimetres of rain during the storm to date.
"It really is widespread across the North Island," she told Radio NZ.
Asked what area gives her the most concern, Ms Murray replied, "There are so many areas ... there's a lot of places in trouble."
On the East Coast, the Gisborne and Hawke's Bay regions have been battered by similar totals, with rivers breaking their banks and forcing evacuations.
"I've been in the region over 20 years and this is by far the biggest (storm) ... people have not seen a storm like this. It's a very, very significant event," Hawke's Bay civil defence spokesman Ian Maxwell told Radio NZ.
In Muriwai, grave fears are held for the safety of the volunteer firefighter missing after the home's collapse.
Fire and Emergency NZ chief executive Kerry Gregory said the property was too unsafe to search.
"Our thoughts are with our firefighters, and with their loved ones. We are also providing support to the other members of their brigade," he told TVNZ.
Transport authority Waka Kotahi reports about 50 road closures, including State Highway 1 in several places and key arterial roads in the Coromandel and East Coast.
Cyclone Gabrielle will continue to wreak destruction across North Island on Tuesday.
Red wind warnings remain in place for Auckland, Northland, the Coromandel and Taranaki, with gusts of up to 140km/h forecast.
Red rain warnings have been extended in Northland, the Coromandel and Hawke's Bay.
A majority of the North Island, including Wellington and Napier, are under orange warnings for wind, suggesting up to 120km/h gusts, as is the northern region of South Island.
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