WELLINGTON —  A coronavirus cluster in Auckland has risen to 17 cases, New Zealand health officials said on Wednesday, raising the prospect of an extended lockdown in the country's biggest city to battle the resurgent virus.

National health chief Ashley Bloomfield said there were 13 new confirmed infections, all linked to four family members found on Tuesday, ending New Zealand's record of 102 days without community transmission of the disease.

Police have been helping to implement a three-day lockdown in Auckland since midday Wednesday as teams of health workers raced to find the cluster's origin and ramped up testing in the city.

Bloomfield said among the new cases was a student at one of New Zealand's largest high schools, attended by more than 3,000 children.

Prime Minister Jacinda Ardern said more cases linked to the recent spike were expected.

"As we all learned with our first experience with COVID, once you identify a cluster, it grows before it slows," she said in a televised address.

Ardern said she was encouraged by the fact that all the cases so far were in a single cluster limited to Auckland.

"You can see the seriousness of the situation," she said.

"While it is serious, it is being dealt with in an urgent but calm and methodical way."

Bloomfield said authorities were urgently exploring all possible routes of transmission.

"We want to find out how large it is as soon as possible, so we've been testing all close contacts, casual contacts, workplace, family-related," he told TVNZ.

"This is what we want to do as quickly as possible to find out how extensive the outbreak is and who the first case might have been."

He said any decision on extending the lockdown depended on what the investigations uncovered over the next 24 hours.

"It's too early to say... we'll have a lot more information tomorrow," he said.

He played down one line of inquiry examining whether the virus was imported via freight, then picked up by a male member of the family, who worked in a cool room for imported goods.

"It's a possibility - it's unlikely but it's something we need to rule out," Bloomfield said, adding that another focus was whether the infection came from managed isolation facilities.

Despite New Zealand's previous success in containing the virus - with just 22 deaths in a population of five million - Bloomfield said health authorities had always anticipated such a return, even it had taken some people by surprise.

"Yes we were becoming complacent and that's why our message over the last few weeks has been around avoiding that," he said.

"As you'll recall, last week I was talking about when - not if." — AFP