Bluff Oysters are in trouble - with a fatal parasite on the move.

The Ministry for Primary Industries say Bonamia has been found in oyster farms at Stewart Island.

It was found in Nelson and the Marlborough Sounds two years ago - but the latest discovery shows it's spread.

Head of operations at Bluff Oyster Management Company, Graeme Wright, said the effects can be devastating.

He says told Mike Hosking not much can be done to prevent it.

"No level of control at all really, it's really at this stage, it's just trying to do whatever we can to prevent it entering into the fishery," he said.

LISTEN ABOVE AS GRAEME WRIGHT SPEAKS WITH MIKE HOSKING.