Another problem was that there were some people involved who weren't exactly honest as firearms that were handed in were discovered years later in the hands of criminals in differant states. They were found during police raids.
Nick, it's not just a down-under problem. People here talk about how high-capacity rifle magazines should only be used by police or military; well, I read (and saw a picture) in my Sunday paper where the latest LA buy back saw the return of 2 empty rocket launcher tubes. If the military can't keep track of it's large-size hardware and keep that sort of stuff off the street how the heck does anyone think that high-cap magazines will remain only in the hands of the military/police? (Not that that disparity in itself isn't an invitation to problems).