I completed my HSC in 2001, here's what I did
2U Standard English
2U General Mathematics
2U Biology
2U Software Design and Development
2U Information Technology
2U Economics
Attained a UAI of 86.75. Don't stress over the HSC, do not be concerned about what marks you need, just study and be as relaxed as possible. I know its hard to do, but the more stressed you get, the harder you'll find it, and in reality it will hurt your marks.
The HSC isn't the be all and end all (regardless of what your parents or teachers tell you), it isn't the one factor in your life that will determine who you are, it isn't the one thing that will determine your career. I know plenty of blokes that got 90+ in their hsc, only to do nothing with it. I know even more people that got less than 50 and have gone on to do great things and get high paying jobs. If you want to goto university thats great, but if you don't get the entry mark its not the end of the world, there are plenty of pathways into university. At worst not getting the mark you want will set you back 3 years (until you're a mature age student), hardly alot of time in the big scheme of things.
Work hard, be calm, enjoy your final year of school, don't stress your way through it.