Hi Minni
Given that we can only surmise as to exactly WHAT they saw when the body was discovered, we can only go on the rumours. So basically, I can only guess along with the rest of you as to how they deduced it was murder - eg clingfilm or plastic bag over the head, etc. Or else, the whole thing about the only injury being a chipped tooth may be a furphy.
The difference between ante-mortem and post-mortem injuries is not usually obvious on initial examination of a body that has been moved. It can be quite obvious under the microscope, of course, but the reason I mention the movement of the body is that any bleeding or blood spatter analysis would not be possible, which is one way of telling ante- from post-mortem injuries. The amount of bleeding, and the distance of any arterial spray, for example, can give good indications - but not in this case. Also, one would have to take into account any damage caused by creatures such as bush animals if she were place in the bush prior to being washed into the creek, and then water creatures after that.
So I was quite surprised that the police seemed to come out with those statements so quickly. But don't forget that the autopsy was done with very little delay after the body was found, and the body released for burial. So - much of that information may well have been known within the first day or two anyway.