New Zealand troops supply aid to storm-hit towns

New Zealand redeployed gunships and helicopters to deliver water, food and fuel to cities cut off by Cyclone Gabrielle, as overwhelmed rescue teams embraced overseas offers of help.

Five people have died and 10,500 more have been displaced, according to authorities, in a disaster that has crippled the country's populous North Island.

Four days of violent winds and lashing rains caused landslides and widespread flooding that lacerated the island's road network, caused rolling power cuts and knocked out hundreds of mobile phone towers.

Police said 3,500 people had been reported as "uncontactable", as residents around the country tried desperately to reach loved ones.

New Zealand Prime Minister Chris Hipkins said the police held "grave concerns" for those still missing.

"We do need to be prepared for the likelihood there will be more fatalities," he warned.

Hipkins said over 100,000 households were still without power on the North Island, home to more than three-quarters of the country's five million residents.

The east-coast city of Napier was briefly cut off after experts detected damage to the last usable bridge linking its 65,000 residents with the rest of the country.

Isolated residents have been told not to leave their homes unless "absolutely essential" and to restrict water use.

Around the city, petrol stations have put up signs declaring there is "NO FUEL" for anyone other than emergency services.