Authorities warn that the number of cases is likely to increase, which raises the prospect of a three-day shutdown in Auckland and puts the date of the country's upcoming parliamentary elections in doubt.
"As we all learned from our first experience with Covid, once you identify a cluster, it grows before it slows down," said Prime Minister Jacinda Ardern at a news conference on Thursday. "We can expect that to be the case here."
But on Tuesday, New Zealand reported that a family of four in Auckland had been infected with the coronavirus, despite having no history of traveling abroad and no connection to known cases. This led Ardern to reintroduce coronavirus restrictions, including closing access to nursing homes, introducing the Auckland Barrier and restricting gatherings around the country to no more than 100 people.
On Thursday, New Zealand's Director General of Health, Dr. Ashley Bloomfield, announced the 13 new locally transferred cases, all associated with the original four.
It is still unclear what caused the last outbreak. For several months, New Zealand has imposed strict immigration controls – with some exceptions, the border is closed to all non-New Zealanders, and anyone entering the country must spend 14 days in a state quarantine facility.
Authorities have used genome sequencing to investigate the source of the outbreak, but have found no link between civil cases and imported cases in New Zealand's administered isolation quarantine, Bloomfield said. "What we do know is that the genome of the new cases is very similar to the patterns of the United Kingdom and Australia," he said.
One of the original four worked at Americold, a US temperature controlled storage company. On Thursday, the authorities announced that three more employees of Americold had tested positive, and seven of the cases announced on Thursday are family members of employees of Americold.
Authorities are testing surfaces at Americold's warehouse for Covid-19, but Ardern emphasized that the most likely cause of the outbreak was human-to-human transmission. "We want to exclude everything," she said.
New Zealand's new cluster comes just weeks away from a September 19 national election. Parliament was to be dissolved on Wednesday, but Ardern postponed it until August 17 – and she will not yet announce whether the election will go ahead as originally planned.
On Wednesday, Ardern emphasized that Australia's situation – and the situation in countries around the world – showed how important it was for New Zealand to adhere to the new restriction rules.
"Our response to the virus so far has worked, it has opened up our economy and communities, and it has given New Zealand citizens the freedoms we value, and we all want to get back there as soon as we can. But success depends on that we all work together, "she said.
"We know how to beat this, but we also know we do not have to look far to see what it can mean if we do not get on top of it."
CNN's Isaac Yee contributed reporting.