Tasmania's Primary Industries Minister will not say why two expert members of his eight-person Marine Farm Review Panel quit.

Professor Barbara Nowak and Louise Cherrie quit after the Marine Farm Review Panel approved 30,000 tonnes of salmon to be farmed in Storm Bay.

There are allegations that the two experts resigned in protest over the process for approving the expansion of salmon farming in Storm Bay.

The MFRP approved proposals by Tassal and Huon Aquaculture to farm new waters in Storm Bay, boosting the current total output of the area by about 150 per cent.

In its submission to the MFRP, the Derwent Estuary Program (DEP) raised significant concerns.

“This is a very large biomass compared with current salmon production across Tasmania … Estimated nutrient loads associated with these production levels are very large,” it warned.

The DEP says 40,000 tonnes of salmon in Storm Bay would generate 1,892 tonnes of sewage — about six times the River Derwent's current sewage load.

Tasmania's Environment Protection Authority recommended 30,000 tonnes a year as a starting point.

However, there is limited scientific evidence to support the proposition that either amount of farmed fish in Storm Bay would be sustainable and have no unacceptable environmental impacts.

Tasmania's Primary Industries Minister, Guy Barnett, has confirmed he received a letter outlining the reasons for the resignations, but refused to make it public.

“Sorry, it was a private letter from them to me as Minister,” he said.