A nice refreshing punch in the head

No doubt we’ve all seen the footage of Danny Williams clocking Mark O’Neill in the back of the head during Melbourne’s visit to Leichhardt over the weekend. We’ve no doubt all got an opinion on what the penalty should be in terms of the NRL Judiciary. You can read about the incident here. 
 
But where should the boundary lie between actions which can be reasonably expected as part of a full-on contact sport played at the highest level, and actions which really are criminal assault? 
 
The ‘voluntary assumption of risk’ principle would surely rule out criminal (and even civil) action in relation to most on-field punch-ups. But there’s just something about this incident which, to me anyway, precludes any argument that it was just ‘part and parcel’ of contact sport.
 
Personally, I don’t think the fact that they were wearing team uniforms, and standing out on a field somewhere should disguise the fact that this was an act of pure violence, and thus should catch the attention of higher powers than the NRL Judiciary.  Our current criminal code (and civil law as well) pretty much rotates around the principle of the sanctity of one's person. I don't recall there being a blanket exemption in relation to overpaid, over-excited football players.
 
But then again, if O’Neill chose to make a civil complaint, or raise the matter with the Police, I have a funny feeling he would soon be cast as the villain in this story.