Public health officials in Marlborough are warning people not to gather shellfish in Port Underwood, west of Picton, because of a toxic algae outbreak.
The Nelson-Marlborough Public Health unit says the alga is toxic to humans, causing severe diarrhoea, though it does not harm the shellfish.
Several commercial fisheries in the area have temporarily stopped harvesting.
Environmental health manager Neil Silver says it is difficult to say how long the bloom will be in the area.
He says once it goes the shellfish will purge the algae and will be fine to eat again.
Mr Silver says there is no risk from swimming.