It is still not clear which regions will be worst hit by Cyclone Gabrielle as it approaches New Zealand, weather forecasters say.

The storm is due to track across Northland on Sunday before moving south to Auckland, Coromandel, Waikato, Bay of Plenty and Gisborne on Monday and Tuesday.

Forecasters are warning of strong wind, heavy rain and big seas.

MetService meteorologist Lewis Ferris said the cyclone would bring widespread severe weather.

The cyclone had taken a more eastern track over the last few days, but there was still disagreement over where the central point will lie when it made landfall, Ferris said.

"It's still looking like Monday-Tuesday are going to be the biggest days for the weather with the approaching cyclone. The worst impacts, where they are and when they occur, are still going to be riding on where the track of the cyclone actually eventuates."

The storm is due to track across Northland on Sunday before moving south to Auckland, Coromandel, Waikato, Bay of Plenty and Gisborne on Monday and Tuesday. It is possible that even Wellington could see some impacts, MetService said.

It has been upgraded to category 3.

MetService has issued heavy rain and strong wind watches ahead of its arrival.

Meanwhile, the government is urging people to avoid non-essential travel in areas that could be hit by Cyclone Gabrielle.

Air New Zealand is offering flight deferrals in affected areas, asking people to postpone air travel unless it was urgent.

Emergency Management Minister Kieran McAnulty said safety trumped the disappointment of scrapping weekend plans.

"We would urge people, despite the inconvenience this would naturally cause, to heed that advice, because it is not given lightly. We are taking this very seriously. Depending on how this tracks it could be quite severe and we're just asking people to take it seriously."

McAnulty said he had assurances government agencies and local civil defence services would update their social media channels regularly.

People are also being urged to stock three days' worth of food and water and prepare for possible power outages.

Supermarkets have urged shoppers to be patient as people stock up on essential supplies.

St John Ambulance said it was scaling up in preparation for Cyclone Gabrielle.

Spokesperson Dan Ohs said St John had increased deployments in Northland, Coromandel, Auckland, and Bay of Plenty.

There would be extra ambulances and four wheel drives on the ground, he said.

"For us it looks like additional ambulances where that's relevant and in particular, four-wheel-drives. We're also in discussion with Wellington Free (Ambulance). Wellington Free have a rescue unit capability and four-wheel-drive resources."

Parents in Auckland are likely to receive communication from their children's school this weekend about Cyclone Gabrielle.

Yesterday, the Ministry of Education advised Auckland schools that they must decide individually whether to stay open or close in response to the storm.

The decision would be based on schools' assessment of health and safety risk of their situation.

A statement from the ministry said schools did not have to wait for direction from the secretary for education.

However, the secretary did have power to close services based on information from emergency services.

Auckland Secondary Principals Association President Greg Pierce said Covid-19 and the recent floods had prepared to schools to act quickly.

Schools would keep families regularly updated on the storm, Pierce said.

In Northland a number of maraes may be available as welfare centres when Cyclone Gabrielle hits the region.

A health organisation based near Kaitaia, Whakawhiti Ora Pai, said it had been talking to maraes in the area.

General manager Errol Murray said several marae are prepared to step in if needed, including Potahi Marae in Te Kao, Waiora Marae in Ngataki.

Murray said the had also been doing welfare checks on vulnerable communities ahead of the cyclone, as well as preparing food packs and water.

And an iwi in the far north said community support networks developed during the Covid-19 pandemic had made it more resilient to face sudden weather events.

Te Runanganui o Te Aupouri chief executive Mariameno Kapa-kingi said they had already started doing welfare checks and stocked up on supplies and water ahead of the cyclone.

"We did a lot of this work ... communities (in Northland) are still very Covid-aware, right, so a lot of the work we did then is actually carried forward into capacity that we have today to deal with this."