Anti-shark kill protesters who claim to have pulled up a drum line off Cottesloe and dumped it on the beach each face a fine of up to $25,000.
They say they did it "in the spirit of artistic expression" and hope to avoid prosecution.
Beachgoers at the Sculpture by the Sea exhibition on Saturday were stunned when members of the Forest Rescue group, who were painted blue, pulled an inflatable dinghy ashore and deposited a drum line on the sand.
The device had been about 1km offshore as part of the Barnett Government's shark cull.
A spokeswoman for Fisheries Minister Ken Baston said the Government was aware of the incident and was investigating.
Individuals face up to $25,000 in fines and a year in jail for unlawfully interfering with fishing gear or could be hit with a $10,000 fine for tampering with a lawful fishing activity.
Activist Simon Peterffy, who was in the group, said the drum line was a "late entry" into Sculpture by the Sea, though he acknowledged it was taken in protest at the policy.
Mr Peterffy said the group realised interfering with drum lines was illegal but he hoped authorities would see the "artistic" merit in the episode and spare them prosecution.