Its so hard to actually say what marks you need to achieve an above 90 UAI, but these are my marks for my HSC last year
English (advanced): 90
English (extension 1): 38
Mathematics: 91
Extension Maths: 39
Chemistry: 76
Physics 72
2004 UAI: 91.5
As you can tell, even with lower marks in Chemistry and Physics in particular, you can still achieve an above 90 UAI.
Then again, I wasnt happy with my results and made the stupid decision to repeat year 12 again this year... damn courses with required UAI of 98!
So UAI is really equally dependent upon marks AND courses... for some ppl, if you got band 6 for everything, you may get UAI of only just 90, whereas for others a band 6 in everything means UAI of 99+