An investigation is under way on the NSW north coast after a critically endangered grey nurse shark washed ashore with its fins slashed off.

Fisheries officers have searched boats and buildings and interviewed fishermen since the mutilated and dying shark was found on a beach near Evans Head last week.

A critically endangered grey nurse shark on a beach near Evans Head with its fins slashed off.

The grey nurse, a rare young breeding female, had both dorsal fins removed. Only about 1000 to 1500 grey nurse sharks survive off eastern Australia.

"It is illegal to remove the fins from any shark at sea," a spokesman for the Department of Primary Industries said.