Read to this document.
http://www.boardofstudies.nsw.edu.au/hsc/hsc_update_2002_06.html
I'm pretty sure they are revised after the marking process to ensure everything has been properly addressed, but I think the majority of the decisions made are before the exam by experienced judges.
Judges review and discuss samples of student examination scripts awarded marks equal to or near each cut-off mark the judges are proposing.
The judges read and discuss these scripts with a view to determining whether or not these works are typical of what they would expect of students at that level. In essence, they need to satisfy themselves that the scripts scoring the cut-off marks can be said to match the standards demonstrated by those student works in the standards packages.
During this process each judge has the opportunity to further refine their band cut-off marks.
At the completion of this process, judges recommend to the HSC Consultative Committee a set of examination marks corresponding to the cut-offs between bands.