Crew members aboard a container ship currently at sea off the coast of Tauranga have tested positive for Covid 19.
Eleven of the 21 crew on board the Rio De La Plata are confirmed to have the virus.
Public Health staff took the test swabs in Tauranga from the crew as part of the requirements for entry for the vessel to its next stop - Napier.
The Ministry of Health said in a statement that it was likely at least some of the 11 crew were active cases of Covid-19 and further test results, expected today, would help determine how many of the crew were likely to be historical cases and no longer infectious.
All crew on board are reported to be well, with none reporting any symptoms, the ministry said. No crew members came port-side while the ship was being unloaded in Tauranga.
The ship is linked to a Covid-19 case in an Australian pilot who was on board the ship July in Queensland and who later developed symptoms and then tested positive for Covid-19 nine days after being aboard the vessel.
The Australian pilot is confirmed to have the Delta variant and has not been linked to any other Queensland cases.
The ministry said initial concerns about the link with the Australian pilot resulted in loading of the vessel being temporarily halted on Wednesday 4 August in Tauranga. An assessment carried out that day, cleared the ship, and unloading was allowed to resume the following day.
As well as the usual precautions, 94 port workers who spent time on the vessel while it was in port are being contacted, provided with advice, tested and stood down until a negative result returned.
National Party Covid-19 Response spokesperson Chris Bishop said he was concerned that 94 workers from the Port of Tauranga had been on board the ship given their vaccination rates.
"The potential for spread into these workers and into the community I think is a real one and I think is quite worrying," he said.
"Nearly 100 [port] workers on board the ship; 11 - so around half the workers on the ship - are positive; it is Delta, and of course around about 45 percent of port workers nationwide are unvaccinated."
"You just look at common sense and say that is a real vector of transmission and a real risk."
Covid-19 Response Minister Chris Hipkins last week said he was "very concerned" about statistics showing just 44 percent of port workers across New Zealand had the Covid-19 vaccine, with misinformation playing a role in vaccine hesitancy in the workforce.
Bishop said New Zealand was a sitting duck when faced with the Delta variant.
"Because of our largely unvaccinated population, particularly at our border when it comes to the ports."
Public health expert Otago University Professor Nick Wilson has warned that even a single case of the Delta strain could prompt a lockdown.
Auckland University Professor of Medicine Des Gorman said the weakest part of the country's border was the ports, and crew members should not be allowed off the ship unless they needed to be hospitalised.
"It's a very worrying situation to have a ship full of people who are infected ... interfacing with a port where 40 percent of people aren't vaccinated."