Some 11 cases of Covid-19 have been confirmed at a managed isolation facility in Christchurch, with another 14 possible cases being investigated.
The Ministry of Health says it is investigating after the cases were detected during routine day three testing.
None involved cases in the community, it says.
The ministry says the positive cases are part of a group who are the only people staying at this facility.
Further details will be reported tomorrow, it says.
Stuff has reported several new cases have been detected at the Sudima Christchurch Airport where a number of international fishers are staying.
RNZ reported last week that more than 400 foreign fishers were headed to New Zealand to crew deep sea trawlers after failing to find Kiwis to fill the jobs.
A charter flight from Russia arrived in Christchurch on Friday, where the first 200 crew began managed isolation.
The crew are mostly Russians with others coming from Ukraine. Russia has recorded more than 1.3 million cases of Covid-19 - the fourth highest number of any country.
Seafood New Zealand chief executive, Jeremy Helson, said all the men were tested before they flew to New Zealand.
"All of these fishers were Covid tested before they took the charter flight into New Zealand. All crewmen tested negative. This pre-flight test was beyond what the government required," Helson said.
"While we wait to see how many cases there are, the fact that they were all detected in quarantine shows the system is working well."
Healthcare, isolation stays to be paid by fishing companies - Sealord
Sealord chief executive Doug Paulin told Checkpoint he only knows what has been reported in the media and has not been contacted by the ministry of health or managed isolation and quarantine.
"I imagine that will take place in the near future."
"They'll have a protocol and a process they need to follow and I think the most positive thing we can take out of this is that border protection at work.
A total 237 Russian and Ukrainian fishermen arrived in Christchurch on Friday - to work for Sealord, IFL and Maruha - with 69 of those from Sealord. Paulin said all the workers came in on a private charter flight from Russia and only fishers and air crew would have been on the plane.
Paulin said he had no regrets about bringing the workers to New Zealand.
"This is a significant economic issue for not only the fishing companies themselves, but to a raft of other companies across New Zealand.
"There are no Kiwi workers that can do these jobs. Some of these fishermen have been fishing for 25 years. They have qualifications which take many years to receive and - at this point in time - there are not enough New Zealanders who actually have those qualifications to do those roles."
Paulin said any additional costs such as healthcare or longer stays would be looked after by the fishing companies.