© Jonathan Carroll
DEATH TOLL: Robert Diemar has lost 600,000 oysters in just weeks
Biosecurity experts are scrambling to identify a mystery disease that has decimated Port Stephens' multimillion-dollar Pacific oyster crops and sent several growers to the wall.

There are fears the port may have to be quarantined to prevent the disease spreading.

Hundreds of thousands of Pacific oysters have died since late last year.

''We lost 600,000 oysters over a couple of weeks. We were struggling to find any live ones to be honest,'' veteran Salamander Bay oyster grower Robert Diemar said.

''We haven't seen anything like it before.''

Pacific oysters, which are worth about $3million to the Port Stephens oyster industry, had been recovering from a disease that swept through the region last year.

To date, it appears the latest disease has affected only hatchery-sourced Pacific oysters.

However, it is feared it may also attack prime Sydney rock oyster crops in the port.

''It's very frightening not only for the guys who are growing Pacific oysters, but also for the rest of us,'' Port Stephens shellfish program chairman Don Burgoyne said.

''If it was Pacific Oyster Mortality Syndrome we would have been quarantined by now.''

Several farmers who had invested heavily in Pacific Oysters have been ruined by the disease. ''It's literally taken my business away; I don't know what I'm going to do,'' Andrew Richardson said.

''There's not much we can do but to diversify.

''It's going to take three years for Sydney rock oysters to grow.''

A Department of Primary Industries spokeswoman said biosecurity and fisheries experts were working with growers to identify the cause of the mortalities.

Twenty batches of affected oysters had been tested at the Elizabeth Macarthur Agricultural Institute at Menangle since early last November, with all testing negative for the virus that causes Pacific Oyster Mortality Syndrome.

''It is not clear yet what is causing the mortality event, it could be an environmental factor. In all submissions tested to date, there is no evidence of a disease consistent with a known infectious agent,'' the spokeswoman said.

''Therefore, without any evidence of an infectious disease, the department's biosecurity and fisheries experts are unable to put in place quarantine measures.''

The NSW Environment Protection Authority and the NSW Farmers Association have also been consulted.