The lockdown of more than a million people in New Zealand’s biggest city is to be extended as the country battles to contain its first coronavirus outbreak in months.

Auckland will stay at level three restrictions, and the rest of New Zealand at a slightly more relaxed level two alert, until at least August 26, Prime Minister Jacinda Ardern said on Friday.

Her announcement came after an emergency meeting of the NZ cabinet, sparked by the cluster that started in a South Auckland family earlier this week.

It is the country’s first outbreak in months and had grown to 30 cases by Friday afternoon. News of the first positive tests came just two days after NZ had celebrated more than 100 days without any community transmission of COVID-19.

“In keeping with our cautionary approach and New Zealand’s philosophy of going hard and going early, … cabinet has agreed to maintain our current settings for an additional 12 days, bringing us to a full two weeks in total,” she said.

“Our current expectation is that by this time, the perimeter of the cluster will be identified, will be isolated, and we can move to level two in Auckland with confidence.”

Ms Ardern said health authorities believed the oldest case in this week’s outbreak was in a worker at the Americold warehouse in the Auckland suburb of Mount Wellington, who fell ill on July 31.

NZ’s cluster originated in a worker at this Americold warehouse in Auckland.

Six workers at two of the company’s Auckland cold storage warehouses have since tested positive to the virus.

Confirmed infections have also spread beyond Auckland, with a staff member at a medical centre in the central North Island town of Tokoroa – more than 200 kilometres south of Auckland – also confirmed with COVID-19.

There is a second case in Tokoroa, and an additional probable infection.

One of the centre’s staff is reportedly linked to the Auckland outbreak.

Ms Ardern said contact tracing and genomic testing had so far found no link to the border or the country’s managed quarantine facilities for returned travellers.

The virus also had a different genomic sequence from those in community cases in New Zealand’s first outbreak earlier in 2020.

“This suggests this is not a case of the virus being dormant, or of a burning ember in our community,” she said.

“It appears to be new to New Zealand.”

On Thursday, New Zealand’s Deputy PM Winston Peters told Australia’s ABC TV that he believed the Auckland outbreak was linked to a breach in the country’s quarantine system.

He said he had been given that information by a New Zealand journalist, who he said was “usually very reliable”.

“It wasn’t an official, I found out from somewhere else, but I think there’s been a breach inside our quarantine system,” he said.

On Friday, Ms Ardern again warned that the outbreak was likely to get worse before case numbers fell.

“It will grow before it slows. And it may continue to be linked to schools, churches and social gatherings, as it has done to date,” she said.

“We also know, based on overseas experience and our own, that it is possible to contain a cluster or outbreak without ever being able to identify its origin.”

While the lockdown has been extended until August 26, it will be reviewed on August 21.

The late August end to the virus restrictions brings New Zealand even closer to its national election, which is set for September 19. Kiwis will be able to start voting in the election in three weeks.