Auckland’s COVID-19 outbreak has swollen to at least 21, with confirmation on Thursday of 13 more positive tests.

The numbers climbed as Prime Minister Jacinda Ardern warned New Zealanders the situation was serious and would likely get worse before it improved.

“As we all learnt from our first experience with COVID, once you identify a cluster, it grows before it slows. We should expect that to be the case here,” she said.

Among positive cases identified on Thursday are workers at two refrigerated warehouses in Auckland and a student at one of the city’s biggest secondary schools.

The positive tests take the total number of active cases across New Zealand to 36. There is also another probable case in the Auckland outbreak that is likely to take it to 22.

“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.”

NZ’s other active cases are returned travellers in mandatory quarantine.

Until Tuesday, New Zealand had gone 102 days without a case of coronavirus from community transmission.

The Auckland warehouses are run by US-based company Americold. The company’s Australia and New Zealand managing director, Richard Winnall, said the three people who had tested positive at the company’s Mount Wellington and Auckland airport sites all worked with a man in his 50s who was already confirmed to have contracted COVID-19.

Mr Winnall told the ABC there had been two positive virus cases at Americold’s Melbourne plant in recent weeks.

But he said there could be no link because the Melbourne warehouse does not ship freight to the company’s Auckland plant.

Another seven family members of the first four cases have also been infected. They include a student from Mount Albert Grammar School.

Another case is an employee from the Dominion Road branch of loans broker Finance Now in central Auckland.

Two of their close contacts also tested positive.

Auckland has been locked down until at least midnight Friday. The government will decide at a crunch Cabinet meeting on Friday whether to extend or deepen the lockdown, a decision that will be largely based on case numbers.

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Director-general of health Ashley Bloomfield said all positive cases would be managed in a quarantine facility.

“This is different to how cases were managed at previous levels, and it shows how serious we are,” he said.

Virus testing has also been dramatically stepped up across the North Island.

Ms Ardern said the Auckland outbreak 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.”

Earlier, New Zealand health officials said it was only a matter of time before they got to the bottom of the mystery new outbreak.

Contact tracing, isolation and testing is crucial in NZ which – unlike other countries – is pursuing an elimination strategy of the virus.

New Zealand’s Cabinet must also consider whether to postpone a national election slated for September 19.

-with agencies