Four platypus have been found dead in an 'opera house' yabby trap in the upper Shoalhaven River.
Geoff Williams from the Australian Platypus Conservancy said in New South Wales the law was clear.
Opera house traps are banned in waters where platypuses are found, including all river systems east of the Newell Highway.
"Unfortunately people can buy these traps for a few dollars", Mr Williams said.
"They are rarely provided with clear instructions as to where they should be used. The average customer has no idea that they can kill platypuses and as a result there are numerous by catch incidents, sometimes involving multiple mortalities if the trap has been left in the water for a long time."
Victoria has recently banned the use of enclosed yabby traps completely. It now permits only open-top traps.
The Recreational Fishing NSW Advisory Council has already recommended that a ban on opera house traps be considered, in line with the Victorian initiative.
Dr Tom Grant, of the University of NSW, has been studying the platypus population in the upper Shoalhaven River for many decades, close to where this latest illegal trap was found.
He said that platypus are particularly vulnerable to being killed because yabbies form part of their natural diet.
Once lured into a trap, a platypus will drown in less than three minutes. If a breeding female is killed in this way during the summer months (the most common time of the year for such traps to be set) any dependent babies waiting in the nursery burrow for her return will slowly starve to death.
Anyone who finds an illegal trap should report it immediately to NSW Fisheries 'Fishers Watch' hotline on1800 043 536.