New Zealand has been rocked by the deadly virus' return into the community this week after 102 days without local transmission.

The new positive tests, as well as one probable case, take the Auckland cluster to a likely 22 people.

"The modelling suggests that a cluster like this will grow," Ms Ardern said.

"But it's not a sign that it's escalating beyond our capacity, it's just the behaviour of clusters."

On Tuesday, the discovery of four cases prompted Ms Ardern to announce a 60-hour lockdown the country's biggest city, which began at midday on Wednesday.

Health officials have moved swiftly to locate, isolate and test close contacts since then.

For the first time since the pandemic's arrival, community cases will be asked to relocate to mandatory isolation facilities previously used for arrivals at the border.

One of the new positive cases confirmed on Thursday is a student at Mount Albert Grammar, the country's second-largest school with 3000 pupils.

The student attended class on Monday, and local health authorities said they were contacting and isolating around 100 close contacts they had while at school.

There are also four new cases of employees at two Auckland businesses and eight family members of those employees, all linked to the index case.

Ms Ardern said the cluster was "serious but being dealt with in urgent but calm and methodological way".

"Last time, some of our clusters got up into the 80s and 90s," she said.

"The advantage we have here is the fact that we're already in level three (lockdown) restrictions.

"We are moving very, very quickly. We are dealing with one (cluster) rather than multiples. But we are expecting growth so I do think it's important we prepare people for that."

The contact tracing, isolation and testing process is crucial in New Zealand which - unlike other countries - is pursuing an elimination strategy of the virus.

It proved stunningly successful back in autumn, when a 51-day lockdown eradicated the virus from the community and allowed Kiwis to live lives free of any restrictions, aside from border controls.

Since then, any cases in NZ have been identified within the mandatory isolation regime at the border.

That was until Tuesday, when the first positive cases in the community rocked Aucklanders.

Dr Bloomfield is using genomic testing to discover the origins of the outbreak.

"We will find the source, I have no doubt about that," he said.

The new regionalised response means for the first time police are operating checkpoints preventing non-essential movement in and out of Auckland.

Ms Ardern said 17,000 vehicles had been stopped in the first 19 hours of the lockdown, with just 312 turned back.

The government will decide at a crunch Cabinet meeting on Friday whether to extend or deepen the lockdown, a decision largely based on case numbers.

They must also consider whether to postpone the national election slated for September 19.

Finance Minister Grant Robertson said the government would also look at new economic assistance as needed.

Australian Associated Press