Salmon producer Tassal has been accused of covering up a major fish kill in Macquarie Harbour.

Documents obtained by the ABC show Tassal did not report a rise in fish deaths within the required time frame under its licence with the State Government.

Tassal and the Department of Primary Industries, Parks, Water and the Environment (DPIPWE) reportedly fought for two years to keep the documents hidden.

They suggest Tassal notified DPIPWE in February 2015 of mortalities of more than 0.25 per cent a day in pens in Macquarie Harbour.

The exact number of fish that died was not revealed.

Environment Tasmania has called on Tassal and the Government to release information about the number of fish deaths.

The conservationists say there may also be a ‘slow drip kill’ going on in Tassal's Macquarie Harbour pens, where smaller amounts of fish die day by day over an extended summer.

Greens leader Cassy O'Connor called for transparency.

“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,” she said.

Environment Minister Matthew Groom says he knows nothing.

“I don't have the specific details in respect of that particular event but I am happy to follow them up for you, but we recognise there have been some issues at Macquarie Harbour,” he said.

Tassal says it did not breach any licence conditions and would not comment further, while the DPIPWE has not commented yet.