WELLINGTON, New Zealand >> Health authorities said Friday they were investigating whether shipping workers were a source of New Zealand’s first domestic coronavirus outbreak in more than three months.

The outbreak has grown to 30 people and extended beyond Auckland, the country’s largest city, for the first time.

Prime Minister Jacinda Ardern is scheduled to announce later whether a three-day lockdown in Auckland would be extended past Friday, which seemed likely given the increase in cases.

Until the cluster was discovered Tuesday, New Zealand had gone 102 days without community-spread infections, with the only known cases all travelers quarantined after arriving from abroad.

Health officials don’t believe the virus was lurking undetected in the community for months but believe it started with a returning traveler or a border worker. But stumped officials have yet to discover the origins.

All but one of the 30 cases in the outbreak are linked, most through family or work connections. The only known infections outside Auckland were two people in the central North Island town of Tokoroa who were linked to the Auckland cluster by a family visit. Officials said they thought the chances were low the virus would spread further in Tokoroa.

Director-General of Health Ashley Bloomfield said they had only started investigating the 30th case, which was the most recent found and also the first person in the outbreak to be hospitalized.

Several of those infected work at an Americold food storage facility in the Auckland suburb of Mt. Wellington. Officials are looking at the possibility that workers on a freight ship or at the port may have spread the infections, despite physical distancing requirements at those sites and orders preventing ship workers coming ashore.

Bloomfield said they were doing rigorous testing at Auckland’s port “as part of our investigation just to follow that chain of the Americold goods that might have come in through the port and been transported to that Mt. Wellington depot.”

Officials are also investigating the possibility the virus could have survived from abroad on chilled or frozen food boxes and then infected workers in New Zealand, a scenario they consider unlikely.

Bloomfield said that they completed a record of more than 15,000 tests Thursday and they were getting a clearer picture of the outbreak’s contours.

“All of the cases so far, at this point, are connected. They are all part of one Auckland-based cluster,” said Health Minister Chris Hipkins. “And that is good news.”