New Zealand officials are investigating the possibility that its first Covid-19 cases in more than three months were imported by freight, as the country's biggest city plunged back into lockdown.

The discovery of four infected family members in Auckland led Prime Minister Jacinda Ardern to swiftly reimpose tight restrictions in the city and social distancing measures across the entire country.

The source of the outbreak has baffled health officials, who said they were confident there was no local transmission of the virus in New Zealand for 102 days.

"We are working hard to put together pieces of the puzzle on how this family got infected," said Director General of Health, Ashley Bloomfield.

Investigations were focusing on the possibility the virus was imported by freight.

Mr Bloomfield said surface testing was underway at an Auckland cool store where a man from the infected family handled frozen food.

"We know the virus can survive within refrigerated environments for quite some time," he told a televised media conference.

The New Zealand unit of Americold Realty Trust, a refrigerated storage specialist based in Atlanta, USA, identified itself as the owner of the cool store.

Americold NZ Managing Director Richard Winnall told the NZ Herald newspaper the infected man had been on sick leave for several days and all employees had been sent home for tests.

Mr Winnall said consumers should not be alarmed as the man would have been wearing PPE gear and gloves while at work.

A Covid-19 test centre in Auckland

A Visiting Professor of Public Health at the University of Bristol has said it is "quite possible" that the cases were imported by refrigerated freight, because viruses can survive very well in such conditions.

Speaking on RTÉ's Drivetime, Dr Gabriel Scally said these reports are not necessarily a cause for worry, but can add to our greater knowledge of the virus.

He said it is great to have this information, as the more we learn about the virus, the better we will be able to control its spread.

China has reported several instances of the coronavirus being detected on the packaging of imported frozen seafood in recent weeks.

The World Health Organization website states there is currently no confirmed case of Covid-19 transmitted through food or food packaging.

However, it also notes that studies have shown that the virus can survive for up to 72 hours on plastic.

Two members of the infected family had visited tourist sites in the town of Rotorua, about three hours drive south of Auckland, while symptomatic, and a third had gone to work at a finance company in Auckland, also while symptomatic.

Mr Bloomfield said four people who had contact with the family were considered likely cases. More than 200 people have been identified as contacts of the family and health officials were prepared to test tens of thousands of people in the coming days, he added.

Residents of Auckland, home to around 1.7 million people, were given just hours to prepare for the return to Level 3 restrictions, requiring people to stay at home unless for essential trips.

"Going hard, going early with lockdown is still the best response," Prime Minister Ardern said. "Our response to the virus so far has worked ... we know how to beat this."

Ms Ardern also delayed a key step toward a 19 September general election - suspending the dissolution of parliament, which usually kicks off campaigning - until next Monday.

A decision on whether the actual poll would be delayed would be announced before Monday, she said.

With around 1,500 confirmed cases and 22 deaths, New Zealand's exposure to the virus remains well below many other developed nations.