On the flip-side, I'll show how School Certificate was all about basic skills, and you need to do alot more for HSC.
School Certificate (2002):
English: 89
Maths: 90
Science: 89
Geography: 90
History & Citizenship (aka Aboriginal Studies): 74
HSC (2004):
English (Adv): 77
Maths Ext1: 65 (73 exam)
Maths Ext 2: 53
Biology: 88
Physics: 84
Economics: 85
2004 UAI: 86.90
Now school certificate was a bludge, I walked into the maths exam with just a pen. I got a calculator about halfway through, and a ruler and protractor in the last 5 minutes (because I had them spare and had nothing better to do, so I went back and redid my estimates I had made).
HSC while not being harder persay, infact possibly easier, but it requires alot more rope learning of the subjects. You need to memorise way more to answer the questions. You can only rely on natural ability and intelligence so much... about as much as these results indicate.