WELLINGTON, Aug 12 (Reuters) – 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 plunged back into lockdown on Wednesday.

The discovery of four infected family members in Auckland led Prime Minister Jacinda Ardern to swiftly reimpose tight restrictions on movement in New Zealand’s biggest city and travel limitations across the entire country.

The source of the outbreak has baffled health officials, who said they were confident there were no local transmission of the virus in New Zealand for 102 days and that the family had not travelled overseas.

“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 zeroing in on the potential the virus was imported by freight. Bloomfield said surface testing was underway at an Auckland cool store where a man from the infected family worked.

“We know the virus can survive within refrigerated environments for quite some time,” Bloomfield said during a televised media conference.

The NZ business of Atlanta, U.S.-based, Americold, a refrigerated storage specialist with operations in the United States, Australia and New Zealand, identified itself as the owner of the cool store.

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

CHINA HAS REPORTED VIRUS ON PACKAGING

China has reported instances of the coronavirus being detected on the packaging of imported frozen seafood.

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.

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

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

The rest of the country was placed back into slightly looser level 2 restrictions. The restrictions will initially remain in place until Friday.

Police set up roadblocks around Auckland to discourage a mass exodus from the city, while supermarkets began rationing the sale of some staple products amid a rush to the shelves. Long queues formed at COVID-19 testing centres in the city.

Ardern said her cabinet will decide on Friday on the next steps with regards to restrictions.