The ACT government is urging residents to report sightings of the enclosed traps, which threaten native wildlife including platypus and endangered fish.

National Parks and Catchments regional manager Brett McNamara said the platypuses were found trapped in a net in the Murrumbidgee River.

He said the use of yabby traps, including Opera House-style nets, around public waterways in the ACT and the eastern half of NSW was illegal.

"Opera House traps are known to catch and drown air-breathing animals such as platypus, turtles, native water rats and birds," he said.

"These traps can also catch and kill the threatened Murray River Crayfish.

"The use of Opera House traps is one of the greatest threats to platypus and is responsible for an increasing numbers of deaths each year."

Mr McNamara urged anyone who found an enclosed yabby trap in a public area to report it immediately.

The nets can be used on private land to catch yabbies, shrimp and minnow.

"There are a number of signs of an illegal trap including rope or string tied close to the bank going into the waterway, a plastic bottle bobbing in a fixed position in the stream or a lonely fishing float or a small plastic ball not moving with the current," he said.

"We are also asking people to be on the lookout for discarded fishing equipment. If you see any, please remove it to prevent further harm to wildlife."

To report an illegal trap, contact Crimestoppers on 1800 333 000 or Access Canberra on 13 22 81.