People are being urged to cook all raw mussels, following an increase in the number of food poisoning cases.
In the past six weeks the Ministry of Health says 26 people have become sick from eating mussels - mostly raw, smoked or partly cooked ones.
Most cases have been linked to commercially grown mussels harvested from a single Coromandel site, which the Food Safety Authority has closed while it investigates.
The bacterium can cause stomach cramps, diarrhoea, nausea, vomiting and fever.
This only relates to mussels bought raw, not those bought in plastic pottles which are already cooked.
The authority's director, Paul Dansted, says it's possible this strain is unusually aggressive and causes illness even with a low prevalence.