A platypus and three threatened crayfish have drowned in an illegal trap in Canberra's south.

A fisherman found the set trap, containing the dead platypus and three Murray River Crayfish, near Kambah Pool.

ACT Parks and Conservation acting regional manager Brett McNamara says the threatened and protected crayfish were between 10 and 40 years old.

"This sort of incident is incredibly disappointing given the platypus is such an iconic species and the Murray River crayfish is threatened," he said.

Mr McNamara says traps have been illegal in ACT public waterways for more than 10 years because they trap and drown animals like platypus, turtles and water rats.

Rangers will be stepping up patrols along the Murrumbidgee River corridor looking for illegal fishing, yabby nets or traps.

Canberrans are urged to report any sightings of enclosed traps, unattended fishing rods or set lines in ACT rivers and streams to Crime Stoppers or Canberra Connect.