The Department of Primary Industries, Parks, Water and Environment has been focusing its testing on areas that have contracted the disease but chief veterinary officer Rod Andrewartha said testing areas believed to be free of the virus would assist in efforts to provide freedom from disease status for some growing areas.

"The two components to the response to this disease are testing to map where the disease is and a standstill on movement of oysters onto and between farms to prevent spread of the disease," Dr Andrewartha said.

Some areas tested so far have returned negative results, however, there is not enough evidence to demonstrate complete freedom or allow the movement of oysters between these regions.

"Industry have been very supportive in ensuring the standstill of all product and equipment, other than oysters for human consumption, is complied with. This is very important in reducing the risk of further spread to areas that may still be free of the disease."

More than 700 samples from around the state had been tested.

"Our surveillance and testing to date has focused on mapping the distribution of disease. Once the general distribution of the disease is known then our surveillance can focus on confirming apparently unaffected areas free of the disease."

The virus is affecting six commercial growing areas including Lower Pitt Water, Upper Pitt Water, Pipe Clay Lagoon, Blackman Bay, Little Swanport and Dunalley Bay.

There is suspicion of infection at Great Bay and more work is under way to determine whether it has been affected by the virus.

Dr Andrewartha said in addition to the commercial areas, the latest test results have also confirmed the disease in a population of wild oysters in the Derwent estuary.

"While it isn't possible to determine how the wild infection has occurred, it is important information as it contributes to our knowledge of where the disease is occurring."