Photographed that at Blackhead. Its obviously vertebrae and ribs/spars embedded in papa rock. That (papa) would make it up to 15 million years old (Pliocene era) Probably marine reptile or some sort of mammal/fish. Although (todays) fish tend to have cylindrical ribs/spars rather than flat? so I'm leaning more towards reptilian..
I'm no palaeontologist so I could be entirely wrong of course