A parasite that can be fatal for flat oysters has been detected in two Stewart Island oyster farms.
Bonamia ostreae has been in the Marlborough Sounds and Nelson for at least the last two years.
But the Ministry for Primary Industries says this is the first time it has been found in another area of New Zealand.
MPI biosecurity surveillance and incursion manager Brendan Gould says there is no food safety issue from eating oysters with the parasite.
He says fresh, good quality New Zealand oysters continue to be safe to consume.
"Bonamia ostreae is a naturally occurring parasite and is difficult to contain and eradicate as it spreads easily through ocean currents and on vessels," he said.
"This particular strain of Bonamia has been in New Zealand since at least 2015. However strains have been here since the 1960s and are present in the marine environment."
Mr Gould said MPI's priority was determining the extent of the issue and limiting further spread.
Extensive sampling to date had not found Bonamia ostreae in Bluff oysters in Foveaux Strait.
He said MPI had reissued a "controlled area notice" legally restricting movements of some shellfish species, including their spat, into and out of Nelson, Marlborough Sounds and Stewart Island.
The notice also legally restricted movements of farm equipment and vessels out of Stewart Island to limit further spread.