Cyclone Gabrielle is the most significant weather event New Zealand has seen this century, Prime Minister Chris Hipkins says, as authorities work to rescue people trapped on roofs and to restore power. 

A state of emergency was declared in New Zealand on Tuesday as Gabrielle brought heavy rain, flooding and gale-force winds across the North Island and forced the evacuation of several beach towns. 

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

Emergency Management Minister Kieran McAnulty said roughly 2,500 people have been displaced. 

Authorities said they were yet to restore communications to crews in the Hawke's Bay region and the power grid operator warned the community should be prepared to go without electricity "for days or weeks, not hours".

The New Zealand Defence force has deployed 200 personnel to offer assistance, including road and air resources to the region to assist with rescues.

Local media are reporting people are still trapped in their cars.

Fire and Emergency New Zealand deputy national commander Steph Rotarangi said many people in Hawke's Bay and Tairwhiti had been cut off and needed help. 

"Helicopters are trying to rescue people but are being hampered by high winds, as has already been pointed out," she said. 

Approximately 225,00 customers have been without power for a significant of time with some will be without power for weeks. 

Transpower has declared a grid emergency following the loss of electricity supply to Hawke's Bay and Gisborne.

It said flooding to the Redclyffe substation, which was likely underwater, had caused widespread outages.

"Until we are able to gain access to the substation, we will not have a clear picture of how long the region will be without power," Transpower said. 

Grocery companies Woolworths and Foodstuff have been forced to close some stores in impacted regions but insist there is enough stock in warehouses.

They are asking people to just buy what they need until roads and supply chains can open.  

Cyclone Gabrielle will track south-east away from New Zealand from Wednesday but the MetService says its impacts will be felt for much longer.

"A continuing focus will be wind," MetService meteorologist Andrew James said.

"We are still expecting gusts in excess of 120kph over parts of the upper North Island, Taranaki and central New Zealand, and warnings are still in place."

Hawke's Bay Civil Defence Controller Iain Maxwell said the cyclone's impact was yet to be fully understood.

"Our focus continues to be on people and safety, and making sure those who have lost their homes have somewhere warm to be, food and water," he said.

"We do need people to understand that there will be a long recovery time — we're talking weeks and months — following what is an unprecedented natural disaster."

Search for firefighter continues 

The search has resumed for a volunteer firefighter who became trapped inside a house that slid down the side of a mountain in Muriwai, a beachside town near Auckland.

A second firefighter was rescued and is in critical condition in hospital.

Local media reported the emergency services crews were investigating damage inside a property when a landslide occurred and the building collapsed.

A geotechnical assessment confirmed the site was secure, allowing for the search to continue.

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