The average BUNYIP of legend tends to eat anyone within reach, so there are very few eye-witnesses to report what it actually looks like.
By all accounts it is large and bulky — reported glimpses vary from big, black and blubbery to huge and hippo-like. There being no known hippos in Oz this must be treated as hypothetical.
The BUNYIP also gulps down clothes, belongings, buckles, buttons and baggage, leaving no traces or even braces. But it can sometimes be heard, and its cry is harsh and horrible — all the warning you need that trespassers will be eaten.