I think one factor that hasn't been adequately raised up in this discussion is it also depends very heavily whether the particular person likes the degree they have chosen or got into, if the degree they got into wasn't one that they enjoyed, i.e. only selected based on parental expectation or atar requirements, then it is unlikely they are going to put the effort required to succeed.
Another point is that unlike the HSC, people in uni often have many other committments, like work, even relationships if you come down to it, and people start to priortise different things, because in uni, a HD average does not increase your opportunity to get a job without any job experience or connections. So in uni, there is a general focus for more rounded development, and hence doing just as well is a relative term, for some, HD is a minimum, while for others, they may be happy with a D or Credit.