Emergency Management Minister Kieran McAnulty says Cyclone Gabrielle has had major impacts across most of the North Island.

Speaking to media at the Beehive after declaring a national state of emergency, he says Tararua District has also declared a state of emergency.

"This is a significant disaster with a real threat to the lives of New Zealanders. Today we are expecting to see more rain and high winds. We are through the worst of the storm itself but we know we are facing extensive flooding, slips, damaged roads and infrastructure."

"This is absolutely not a reflection on the outstanding work being done by emergency responders who have been working tirelessly, local leadership, or civil defence teams in the affected areas. It is simply that NEMA's advice is that we can better support those affected regions through a nationally coordinated approach."

He says National Emergency Management Agency (NEMA) met with local civil defence teams early this morning and heard that a national state of emergency would be beneficial for them.

It allows the government to support affected regions, coordinate additional resources as they are needed across multiple regions and help set the priorities across the country for the response, he says.

"Our message to everyone affected is: safety first. Look after each other, your family and your neighbours. Please continue to follow local civil defence advice and please minimise travel in affected areas. If you are worried about your safety - particularly because of the threat of flooding or slips - then don't wait for emergency services to contact you.

"Leave, and seek safety either with family, friends, or at one of the many civil defence centres that have been opened."

He says iwi, community groups and many others have opened up shelters and offering food and support to those in need.

"I also want to acknowledge that there have been reports of a missing firefighter - a volunteer firefighter - who is a professional and highly trained but left their family to work for their communities and the search continues. "Our thoughts are with the FENZ staff and their families."

Acting Civil Defence Director Roger Ball says we have multiple weather warnings and watches in place and the effects of the cyclone will continue to be felt across the country today.

He says if other regions or areas declare local states of emergency, they will be added to the national declaration.

"Under a state of national emergency, myself as the director and my national controller have authority to direct and control the response under the Civil Defence Emergency Management Act including allocation of resources and setting priorities."

He says no effort will be spared.

"It's not our intent to start micromanaging the operational response on the ground, we continue to have confidence in the civil defence emergency management groups, in the emergency services and in the responding agencies to do the job in the support of their communities."

He says if people are feeling unsafe they can go to civil defence centres and if relocating should let friends and family know where they are going.

"In impacted areas, the advice is to stay at home or to minimise travel, please follow that advice."

"Drive to the conditions, watch out for flooding and debris on the roads. Do not enter floodwaters, do not play in floodwaters."

He says it's important to stay informed even if power is out. RNZ is the civil defence broadcaster and is available on AM and FM frequencies.

Ball says people are fleeing their homes in Hawke's Bay and formal evacuations are taking place, and some have asked for immediate NZDF support. "That is being provided right now, so that would be a good example of where the difference is being made immediately."

He says they will be in close consultation with civil defence groups as to what their needs are, and if they need communications made at a national level "then that will happen".

About 8am when the national declaration assessment was made, the lack of communication from Tairāwhiti was one of the concerns that was taken into account, Ball says. Communication is "severely disrupted", and the telecommunications companies are working to get cell towers working again.

McAnulty says the assessment is the worst of the weather has passed but it's still serious, and the experience of Cyclone Hale which swept to the east then came back means the situation is being monitored closely.

He acknowledged slash in the Tairāwhiti region had been a repeated concern during extreme weather events over the past few years. "I think the Minister of Forestry has indicated a willingness to look into it and see what might be able to be done but it is a real concern that every single time there is an event like this that is a factor that is particularly affecting that community."

McAnulty says there are various areas of concern, particularly Hawke's Bay, Tairāwhiti and Coromandel, and options that may be available normally like helicopters may not be possible.

Power has affected mobile towers in Tairāwhiti and therefore communication is very limited. Satellite phones have allowed the local civil defence to hand important information to the national centre, which is then handed over to RNZ and other radio outlets.

The declaration, signed at 8.43am, applies to Northland, Auckland, Tairāwhiti, Bay of Plenty, Waikato, and Hawke's Bay, and gives the National Emergency Management Agency (NEMA) the power to step in and provide additional support and logistics to local civil defence organisations.

It is only the third national state of emergency in New Zealand's history, the other two being the Christchurch earthquakes and Covid-19.

* Read more: Live weather updates: Cyclone Gabrielle unleashes fury across North Island

National state of emergency: What you need to know

• The New Zealand government has declared a National State of Emergency, to assist in the response to Cyclone Gabrielle.
• The declaration will apply to the six regions that have already declared a local State of Emergency: Northland, Auckland, Tairāwhiti, Bay of Plenty, Waikato, and Hawke's Bay.
• A national state of emergency gives the national controller legal authority to apply further resources across the country and set priorities in support of a national level response.

National Emergency Management Agency advice:

• Put safety first. Don't take any chances. Act quickly if you see rising water. Floods and flash floods can happen quickly. If you see rising water do not wait for official warnings. Head for higher ground and stay away from floodwater.
• Stay at home if it is safe to do so. But have an evacuation plan in case your home becomes unsafe to stay in.
• If you have evacuated, please stay where you are until you are given the all-clear to go home.
• People should stay up to date with the forecasts from MetService and continue to follow the advice of Civil Defence and emergency services.
• Do not try to walk, play, swim, or drive in floodwater: even water just 15 centimetres deep can sweep you off your feet, and half a metre of water will carry away most vehicles. Flood water is often contaminated and can make you sick.

- RNZ.