An Anyuna native from the Northern Territories is said that after seeing the picture in the newspaper he realised that one of the drawings by the indigenous peoples looked like a plesiosaur. I have posted it here. The creature looks to have been disembowelled. Whether that is its intestines next to it or another creature I am not sure. Perhaps someone with more knowledge of the area and culture may know.
Without much evidence to go on I think we could mark this up to misidentification, though the cave painting is intriguing. There are sharks, crocodiles and various other large marine creatures in the waters around Northern Australia that could account for misidentifications. An unevolved prehistoric creature is a very unlikely explanation.