There are concerns with overseas Covid-19 testing after an outbreak at a managed isolation facility. 

A group of 11 fisherman have tested positive for the virus in the Christchurch Sudima MIQ facility, and 14 cases are under investigation.

Sudima chief operating officer, Les Morgan, says he understands many of the fishermen are from Russia.

The boss of Seafood New Zealand says all fishers were covid-tested before taking the charter flight here and all tested negative.

Auckland University professor of medicine Des Gorman told Heather du Plessis-Allan he'd be surprised if that's true.

"It's essential that people be tested before they travel, but they need to be tested by laboratory systems that we trust." 

Fishing companies had chartered two flights to bring the seamen to this country and are also footing the costs of a 14-day managed isolation for the crew members.

The international seamen are employed by three fishing companies: Sealord, IFL and Maruha.

The companies issued a joint statement saying that detection of the infected crewmen showed that control measures at the border were working.

"Our current concern is the health and welfare of our fishers with Covid-19, however we are reassured that MIQ [managed isolation and quarantine] has all the right protocols in place and they are in good hands.

"At this point we are not sure what the impact will be on our fishing companies, however we fully understand and are supportive of all actions required to avoid Covid-19 transmission in the community," the companies said.

"Any additional cost in management of these cases will be borne by the fishing companies, not the NZ taxpayer."

Seafood New Zealand CEO Jeremy Helson said the incoming fishermen were all Covid tested before boarding the charter flight into New Zealand.

"All crewmen tested negative. This pre-flight test was beyond what the Government required. While we await to see how many cases there are, the fact that they were all detected in quarantine shows the system is working well."

Helson earlier said shore-bound vessels would be costing Sealord, Independent Fisheries and Maruha Nichiro tens of millions of dollars.

Bringing the specialist crew into the country was a lifeline for them as they couldn't keep Kiwis employed in processing plants if they can't fish.

Welcome packs were being translated into Russian and the fishing companies were paying for translators as well as chartered flights and room costs.