The animals were discovered over the past six weeks in Albury botanic gardens in southern New South Wales, with the most recent finding coming on Wednesday.
Two of them had been intentionally decapitated with a sharp object, Wildlife Information Rescue and Education Service Murray River secretary Hazel Cook said.
She told Daily Mail Australia she believes the perpetrators are catching the animals about a kilometre away and putting them on display for the public to see.
'There are no waterways that go into the gardens. We do have platypus that are found in the Murray River,' Ms Cook said.
'So someone has caught them and they've taken them to the botanic gardens. To me that sounds like they wanted them to be found. They could have easily just tossed them in the river.'
Ms Cook says it isn't possible that the platypus were killed by another animal.
'The first thing I thought was that everyone was going to think that foxes had done it, but it wasn't,' she said.
'You could see where someone has used an implement to cut and they went into the hard bone of the vertebrae. We can't tell if they were alive if when their heads were cut off. What they did with the head I don't know - it wasn't left there.'
Ms Cook said she believes it is the same person or persons that is committing the heartless act.
(Daily Mail - April 11, 2017)
