Dave85<------
I got it from an HSC marker, very reliable
Although in my trials I got screwed by my school's ultra stingy science department in one question cuz I put down what he taught me...
Jackson<-----
The external component depends how hard this year's HSC is and the internal depends on how well your school does as a whole in the external, so your 72% may either go up or down.
In terms of how they determine what mark corresponds to what band in the HSC exam the exam setters just sit down and determine how many marks a borderline student in each band should obtain in each question, then add up those marks. Eg a borderline band 6 student should have been able to get 14/15 in the multiple choice section in last year's hsc exam. Of course if you only got 12 or something you could still get band 6 by doing well in the other questions and getting the final mark of 81+/100