Hmm, why is this?
From my experience:
1) The outcomes of the work are supposed to be very different. Uni introduces a lot more material than VCE; VCE is an art of perfecting responses and mastering the study guide.
2) The work flow is very different. VCE is based on exams. My experience at uni (I only did UMEP at Melbourne and nothing else, so I don't really know how Melbourne works) is that work is stuffed down your throat each week depending on your subject choice. It might be different in Australia. At the very least, all of my upper division STEM courses here have 10-20 hours of homework each week, which has to be submitted and counts for 30-40% of my grade.
3) Uni is somewhat of an equalizer once you get in. In high school, privilege plays a huge role: top scores are dominated by people who went to the elite private schools and a handful of selective schools. Once you're at uni, everyone has the same classes and basically the same resources (the biggest difference I can see is how many hours you might need to work at a part time job).
4) A lot of what you do at uni isn't related to coursework. Getting a good internship in engineering or finance doesn't always require the best grades - you need extra curriculars, a strong academic record and to have good people skills. Likewise, opportunities for research/other fellowships after you graduate will depend on how good your research as an undergrad is, which is often not correlated to how good you are at class. So how successful you'll be isn't always linked to your grades. The biggest exception I can see for this is the graduate MD, where grades/GAMSAT matter a lot, and where a job is nearly guaranteed in medicine after graduation.