"Under the Fisheries Act 1995 offenders detected, possessing or using an opera house net in or close to inland waters can be issued with a penalty infringement notice or charged under summons to appear in court and face a maximum fine of $11,300. Vessels and vehicles may also be seized. Under the Wildlife Act 1975 the maximum penalty for killing a platypus is $5,500 and/or six months imprisonment."
I remember reading about one of them even catching a blue billed duck once. I'd imagine they'd catch water rats all the time too.