Environmentalists say Tasmania's biggest salmon producer appears to have covered-up a major fish kill in Macquarie Harbour.

The Greens and Environment Tasmania want Tassal to come clean about what happened in summer 2014-2015.  Their concern comes after the ABC obtained the documents under a Right to Information (RTI) application which show that Tassal failed to report a rise in fish deaths to the State Government within the required time frame under its license.  Both Tassal and the Department of Primary Industries, Parks, Water and the Environment (DPIPWE) objected to that release.

The documents reveal that Tassal gave vague information to DPIPWE about fish mortalities of more than 0.25 percent a day in pens at one of its leases in Macquarie Harbor, but didn't disclose the number of dead fish.

"There is too much secrecy here, too little information, and too much contempt for Tasmanians who want to genuinely see this as a world's best practice industry but they don't want to see it spoil the marine environment," said Tasmanian Greens leader Cassy O'Connor.  

"Environment Tasmania is concerned that Tassal could be concealing evidence of a major fish kill in Macquarie Harbor," said Laura Kelly of the activist group.  Environment Tasmania is also considering legal action over the delay in releasing information.

Tassal maintains it dod nothing wrong.  Tasmania is referring inquiries to DPIPWE.