A Marlborough Sounds oyster farmer says a toxic bloom has stopped him harvesting oysters for nearly a month.
Nelson-Marlborough public health officials say the paralytic shellfish poisoning toxin (PSP) has reached dangerous levels in the Queen Charlotte Sounds.
A ban on taking shellfish has been issued there, as well as in neighbouring Port Underwood where a different toxin was discovered in mid-March.
Bruce Hearn, who farms oysters in Tory Channel, says the PSP toxin outbreak is the worst he can recall.
He says the mussel industry is much less affected, as harvesting is continuing elsewhere in the Marlborough Sounds.