Cyclone Gabrielle is bringing a 'dangerous combination of high winds and heavy rain' today, says Minister for Emergency Management Kieran McAnulty.

He and Acting Civil Defence Emergency Management acting director Roger Ball gave an update on the national response for Cyclone Gabrielle.

McAnulty says the impacts of Gabrielle are now being seen with heavy rain and severe wind in the upper North Island including with damaged roads, downed trees and power outages.

"MetService has advised me that today is the critical day in the event.

"Please keep up to date with MetService for weather warnings, and your local Civil Defence Emergency Management group for local updates."

He says the need to declare a national state of emergency is being monitored and actively reviewed every four hours.

"We have not reached that point and we may not have to. This is an all of government response with all agencies ready to respond as needed."

A state of emergency has been declared in Northland, Auckland, Thames-Coromandel and this morning Ōpōtiki and Tairāwhiti.

McAnulty acknowledges the work of all involved, and thanks everyone for listening to official advice. He says people preparing should only buy what they need - three days' worth.

Roger Ball from NEMA says it is an "unprecedented and dangerous weather event".

"Please stay safe and don't take any chances. We are expecting very high winds and heavy rain today. At 1 this morning, the centre of Cyclone Gabrielle lay approximately 230km nor-nor-east of Cape Reinga."

He says if you find the winds dropping this evening, it does not mean you are through the worst.

"The winds will increase again and come from the opposite direction so you need to be ready to protect yourself and your family throughout tomorrow. Make sure you've prepared your whare for high winds and heavy rain."

He says there are several road closures and advises people who do need to travel to drive to conditions to watch for debris and flooding.

"Please remember, never drive through floodwater."

Northland experienced winds up to 130km/h, Hauraki Gulf 135km/h, and maximum wind speeds at the Harbour Bridge were 110km/h.

Whangārei and Kaikohe received 70mm of rain over 12 hours, with no significant rainfall in Auckland overnight.

Western Coromandel peninsula was buffeted by winds up to 160km/h with speeds up to 110km/h in other areas of the peninsula.

Some 24 civil defence centres and shelters were opened in Auckland overnight, including three mass evacuation centres. He advises people to check with local civil defence for details.

"We understand we have about 58,000 people across the top of the North Island without power. We are working closely with the relevant companies to understand how quickly that can be restored but the indications from those power companies are that it could take some days in some cases."

Live blog of national coverage:

- RNZ.