New Zealand has reported 13 new community infections of Covid-19 as the country attempts to trace the source of its first outbreak in more than three months.

The resurgence of coronavirus comes just weeks before a scheduled general election, increasing pressure on Prime Minister Jacinda Ardern to bring the outbreak under control amid growing criticism.

She had sought to eradicate the virus with a hard lockdown.

Ms Ardern swiftly reimposed tight movement restrictions in Auckland and social distancing measures across the rest of the country after the discovery this week of four infected family members in the city.

"Once again we are reminded of how tricky this virus is and how easily it can spread," she said during a televised news conference. "Going hard and early is still the best course of action."

Officials reported 13 new cases in the community today, and one overseas arrival who had not left quarantine since arriving in the country, bringing the total number of active cases to 36.

Ms Ardern said it was a positive sign that all 13 new community cases were linked back to one infected family, either via work or broader family connections, and that all were being transferred into quarantine facilities.

She said that experience showed "things will get worse before they get better", and more cases were likely to be reported in coming days.

Director General of Health Ashley Bloomfield said health officials were getting "closer every hour" to finding the patient zero of the outbreak.

Mr Bloomfield raised the possibility that the virus had arrived in New Zealand via freight, given one of the initially infected family members works in a cool store that takes imported frozen goods from overseas.

Ten of the new cases were either cool store staff or their family members. The cool store is operated by the New Zealand unit of US-based Americold Realty Trust.

Mr Bloomfield said that importing the virus via freight was considered "a low possibility" as the infection source, but officials wanted to formally rule it out. China has reported the virus on frozen food packaged in South America.

Prime Minister Jacinda Ardern

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Some prominent local health experts suggested it was more likely the virus had been quietly spreading in Auckland for weeks, despite Ms Ardern's efforts to eradicate it with an initial five week hard lockdown.

This possibility has allowed the major opposition National Party to seize on potential flaws in her approach ahead of the 19 September election, which she described as a "Covid election" when launching her campaign.

Ms Ardern is on track to easily win, according to recent polls, but the National Party is pressuring her to delay the vote until at least November, arguing their inability to campaign gives Ms Ardern an unfair advantage.

She said she will announce a decision on the election date after she reveals tomorrow whether lockdown measures will be extended. The election must be held by 21 November.

Critics are also highlighting the economic impact of an extended shutdown.

Reserve Bank of New Zealand Deputy Governor Geoff Bascand said the resurgence posed "a major risk" to the bank's outlook, given its baseline scenario has an assumption that the virus is contained in the country.

1.7 million under lockdown in Auckland

There were long queues for Covid-19 testing centres in Auckland

In Auckland, home to around 1.7 million people, residents were given just hours to prepare for the return to increased restrictions yesterday, requiring people to stay at home unless for essential trips.

Police set up checkpoints at the city's borders as scores of people attempted to leave to avoid a sustained lockdown. Ms Ardern warned they would be turned back.

Long queues of people formed at pop-up clinics, with some waiting hours to be tested for the virus, and there was a rush to supermarkets to stock up on essentials.

There was also growing anger at revelations that members of the infected family had visited several tourist sites in the town of Rotorua and an aged care home in the Waikato district, both south of Auckland.

"We can see the seriousness of the situation we are in," Ms Ardern said. "It's being dealt with in an urgent but calm and methodical way."

With 1,238 confirmed cases and 22 deaths, New Zealand's exposure to the virus remains well below many other developed nations.