There are concerns for Bluff oyster fisheries after a number of oysters in the Foveaux Strait were confirmed to be infected with the parasite Bonamia ostreae.
Biosecurity New Zealand said routine testing had found three infected wild oysters near Stewart Island, with more testing to begin immediately to understand "if and where else the parasite is".
Bonamia ostreae was first found in New Zealand in 2015 in oysters from the Marlborough Sounds and then in 2017 in oysters in Big Glory Bay in Stewart Island, which led to millions of flat oysters being destroyed.
Although it is not a food safety issue, it is described as a "serious concern for New Zealand flat (Bluff or dredge) oysters, including the Bluff fishery".
Biosecurity manager Dr Cath Duthie said the infected oysters were not found in areas where fishing vessels had been working in the current season, and Bluff oysters remained safe to eat.
"Commercial Bluff oyster fishers have not reported any sign of the disease and we've not detected it in five years of six-monthly surveillance sampling in the area," Duthie said on Thursday.
"No commercial oyster fishing vessels have been working the affected area so far this season."
She said Biosecurity New Zealand would be working closely with Bluff oyster fishers, iwi, other marine users and the wider public over the coming weeks to decide on "the most appropriate and effective response to the situation".
"We've talked to fishing industry partners today and will be in Southland tomorrow to start talking to the local community. We want to involve local people, including the iwi of the area, in our response.
"Our immediate focus is to carry out more testing in Foveaux Strait to understand if and where else the parasite is, to enable the best future decisions."
After the parasite was first discovered in 2015 an extensive programme was launched to protect oyster populations from the disease.
Despite that, authorities always knew it was possible the parasite could be found again in wild oysters, Duthie said.
"Foveaux Strait is a dynamic waterway with strong currents and the parasite can remain dormant.
"For this reason, we commissioned NIWA to do six-monthly surveillance sampling and testing of wild oysters from six different locations in the Strait to find any infections at the earliest possible time. We received a confirmed positive laboratory result for Bonamia ostreae from the three oysters today."
Biosecurity New Zealand said Bluff oyster quota holders, who are in the middle of this year's oyster season, will be allowed to continue to fish, with authorities working with them to find the best course of action - which would most likely to be a small temporary and voluntary 'no take' zone around the area where the infected oysters were found.