Taranaki's Medical Officer of Health says it is too risky to eat shellfish from Waitara, after scientists found mussels contaminated with the stomach bug norovirus.
The virus was found during testing for a resource consent application by New Plymouth District Council, which wants to continue piping Waitara's partially-treated sewerage into the sea.
One sample had moderate levels of human strains of norovirus - four others had low levels.
The Medical Officer of Health, Dr Greg Simmons, says the readings are not reassuring.
"The issue with norovirus is that the infectious dose, in other words, the amount of viral particles you need to consume in order to develop an illness is quite low, so there is a potential risk there. That risk to me would be unacceptable, and I wouldn't expect other people to expose themselves to that risk."
Dr Simmons ordered the council to erect signs on Waitara's beach advising of the hazard.
And he says that although he hasn't heard of any norovirus outbreaks, the symptoms of the illness, such as nausea, vomiting and diarrhoea, often go unreported.