Internal marks are determined primarily by a students position in their cohort - a persons position is made up of their ranking and the relative gaps between them and other students. It's also determined by the cohorts performance in the final exam. What happens is the person coming first internally receives the highest external mark from their cohort as their final internal mark - the same goes for the person coming last. Those in between receive a moderated mark based on their rank and the relative gaps between them and other students (so if second isnt far off first internally, then the final internal mark will reflect this).
So final internal marks can only be determined once the externals have been marked.
For a more detailed explanation, here is the official BoS explanation of the process
http://www.boardofstudies.nsw.edu.au...oderation.html.