As far as I know, the university you graduate from does not affect your internship location at all. Thus, students from some unis (UNSW, USYD, WSU) have banded together and made the stack to try improve their chances at getting the oversubscribed Sydney hospitals. If you want to intern at a rural/regional hospital, this doesn't matter at all as there is a separate stream for internship preferencing.
If you change your mind and would like to intern at the big Sydney hospitals, surely you could ask a friend from a Sydney uni, otherwise it does get leaked. I wouldn't be so sure the stack is that effective or that big of a deal anyway, many people still get lower preference hospitals with it.