Nelson-based Cawthron Institute reported finding the single-celled organism Perkinsus olseni last month.
The Marlborough Express understands the affected mussel came from a marine farm in the Marlborough Sounds before being taken to Cawthron for breeding last year.
In accordance with international agreements the find was notified to the World Animal Health Organisation, which has 180 member nations.
A Ministry for Primary Industries spokesman confirmed yesterday that Canadian authorities had placed an interim restriction on the import of live New Zealand greenshell mussel products.
However, he played down the significance of the ban.
New Zealand had exported one consignment of about 88 kilograms of live greenshell mussels to Canada worth less than $1000 since November last year, he said.
Imports of frozen greenshell mussels, which represented the vast majority of New Zealand's greenshell mussel trade with Canada, were not affected by the temporary restriction, he said.
The restriction by Canadian authorities followed the detection of a single organism of Perkinsus olseni in a mussel confirmed in early September, the spokesman said.
"P. olseni is endemic in New Zealand with no recorded mortalities in any shellfish.
It does not present a food safety risk, and no particular production or health impacts are expected from P. olseni."
The ministry was speaking to Canadian authorities about the interim restriction, he said.
The parasite has only been found in warmer northern water and never in green-lipped mussels before.
When it was found last month, the ministry said Perkinsus olseni posed an extremely low overall risk given that it was found through routine sampling and health testing of mussels and not because of any health concerns.
"Prior to the detection, there had been no ill-health or unusual mortalities found in any shellfish at the Cawthron facility."
It was thought no production or health impacts would result from the find.
The parasite had been found in other New Zealand shellfish and could have spread from moving shellfish or from biofouling and ballast from ships, the ministry said.
Both of these actions had been unregulated for a long time.
"There has been no mortality or any sign of ill-health in green-lipped mussels related to this organism and there are currently no tools for eradicating P. olseni from the South Island or other New Zealand waters," the ministry said.