Authorities are investigating how tens of thousands of fish washed up along the East Coast during the past week.

Species include leatherjackets, flathead, salmon and one broadbill swordfish.

Break O'Day councillor John McGiveron, Tasmanian Game Fishing Association president, said the fish, some still alive, had washed up along the coast from Seymour to the top end of the Bay of Fires.

But he said the issue might be more widespread because fish might be washing up in unpopulated areas.

Cr McGiveron said many of the fish were juveniles and the problem might have serious implications.

The Department of Primary Industries, Parks, Water and Environment is investigating.