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New Zealand was acclaimed a world leader in handling Covid-19. Now it's dealing with a fresh outbreak

(CNN) New Zealand reported 13 new community coronavirus cases on Thursday as the country tackles a fresh outbreak that ended an enviable run of more than 100 days without any locally transmitted infections.

The new cluster, which now totals 17 cases, has prompted the country to put its most populous city under lockdown as authorities scramble to trace the source of the outbreak. New Zealand now has 36 active infections, including imported cases. In total, the country has reported 1,238 confirmed cases and 22 deaths.

Authorities are warning that the number of cases are likely to increase, raising the prospect that a three-day lockdown in Auckland could be extended and putting the date of the country's upcoming general election in doubt.

"As we all learned from our first experience with Covid, once you identify a cluster it grows before it slows," Prime Minister Jacinda Ardern said at a news conference Thursday. "We should expect that to be the case here."

New Zealand's outbreak is a dramatic turn of events for the country, which was heralded as a world leader in how it handled the outbreak. For months, life was largely back to normal, and the country went 102 days without a locally acquired case.

But on Tuesday, New Zealand reported that a family of four in Auckland had been infected with coronavirus, despite having no history of overseas travel and no connection with any known cases. That prompted Ardern to reintroduce coronavirus restrictions, including shutting off access to retirement homes, imposing the Auckland lockdown, and limiting 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 transmitted cases, all connected with the original four.

It remains unclear what caused the latest outbreak. For months, New Zealand has imposed strict immigration controls -- aside from 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 the community cases and imported cases in New Zealand's managed isolation quarantine facilities, Bloomfield said. "What we do know is that the genome sequence of the new cases broadly most closely resemble the patterns from the United Kingdom and Australia," he said.

One of the original four worked at Americold, an American temperature-controlled warehouse company. On Thursday, authorities announced that three more Americold staff members had tested positive, and seven of the cases announced Thursday are family members of Americold employees.

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 are wanting to rule everything out," she said.

New Zealand's new cluster comes just weeks away from a national election scheduled for September 19. Parliament was set to be dissolved on Wednesday but Ardern postponed that to August 17 -- and she is yet to announce whether the election will go ahead as originally planned.

The country will be hoping it can avoid a similar situation to its neighbor Australia, where the state of Victoria is reporting hundreds of new cases each day. The state currently has more than 7,800 active cases, and has reported at least 275 coronavirus deaths, as of Thursday. Like New Zealand, Australia imposed strict border controls, but the current outbreak has been linked to poor practices at the border.

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 stick to the new restriction rules.

"Our response to the virus so far has worked, it opened our economy and our communities, and it gave New Zealanders freedoms that we cherished, and we all want to get back there as soon as we can. But success relies on us all working together," she said.

"We know how to beat this, but we also know we don't have to look far to see what it can mean if we don't get on top of it."

CNN's Isaac Yee contributed reporting.