The rays were found with their tails cut off on beaches in Queensland, where Irwin died last week, The Telegraph of London reported.
"We will not accept and not stand for anyone who's taken a form of retribution. That's the last thing Steve would want," said Michael Hornby, the executive director of Irwin's charity, Wildlife Warriors. "We have to protect wildlife more than ever. This is what Steve was about."
Rick Symons, of Queensland's fisheries department, said the deaths were being investigated.