first of all, the exam is finished, if you have more exams, worry about them. if not, enjoy post-hsc life.
only one question, were your school's ranks close at the top?
if so, there's a good chance that you can get pulled up a couple marks, because someone from your school might have had a better day. that can pull your internal mark considering your ranked first. Your estimate for your exam mark shouldn't be too far off the b6 cutoff.
So stay optimistic and hope that everything works out
Based on the way ATAR works i would say that with an internal mark of 90% and a HSC exam mark of 83% or above you are sure to get a band six. As opposed to the previous UAI system which foucised more on the exam mark, ATAR has a heavier correlation to your internal mark. Congratulations mate, you are sure to get a band six with the above mentioned marks.