Director-General of Health Ashley Bloomfield says genomic sequencing shows a clear link between a health worker that tested positive for Covid-19 over the weekend and fishing crew staying at the Sudima Hotel.
The health worker got tested on Sunday after falling ill on Saturday and was revealed to have Covid-19 on Monday evening.
The Ministry of Health announced on Tuesday a second worker had tested positive for Covid-19 since then, a close contact of the first worker.
Dr Bloomfield said further tests had made it clear the first worker's infection was likely linked to the fishing crews.
"It's exactly the same genome sequence as the genome sequence on five of the crew that we had already tested."
Around 200 Russian and Ukrainian fishing crew arrived in NZ in October and 31 have since tested positive.
Nurses' union voices concerns
The NZ Nurses Organisation is calling for an urgent review of how the Covid-19 isolation hotels are being run after the health workers tested positive, saying staff are becoming increasingly anxious and fearful.
Kaiwhakahaere for the Nurses Organisation, Kerri Nuku, says nurses are not given enough support for the numbers of people they have to deal with.
"These nurses are tired, there are few nurses that have responsibilities for large numbers of clients and so the expectation on them is huge."
Dr Bloomfield told Morning Report they're still trying to establish how two members of the healthcare team got infected, but a thorough investigation of the particular case is underway.
"We do a really thorough review of every single case and of where we get an infection like this."
But he's not aware of problems with staffing.
"I'm not aware of any staffing issues per se in either the facility in Christchurch, or shortages in the managed isolation facilities around the country."
Dr Bloomfield said it is important the concerns of nurses are heard and the DHB and public health unit need to sit down with workers and the NZNO to find out what the issues are.
