I've checked UNSW scholarships website, and there are some interesting guidelines:
Eligibility
NSW and ACT High School Principals will be asked to nominate the School's top student, based on the average of available academic results for years 11 and 12.
The number of places is limited to one per school, and nominations must be received by UNSW before end of September each year.
Nominees will receive a Certificate of Recognition from UNSW. The Certificate is not an offer to study at UNSW.
Nominees shall be selected for admission to UNSW based on standard practices (UAI).
If the selected nominee is not made an offer or elects to attend an alternative establishment, the High School may not put forward a second nominee.
If the selected nominee is made an offer to UNSW but chooses to defer, the High School may not put forward a second nominee.
This scholarship cannot be deferred (if you accept your enrolment at UNSW and defer the first year of your studies the offer of the scholarship will be withdrawn)\
It doesn't make it clear, however, if the person who gets the grant is the actual UAI dux of the school. Nominees are due in September, before the actual HSC. So possibly, there could be a winner who was nominated by the school, who say crashes in the HSC and doesn't dux overall who gets the grant. Interesting.
Also, since the school can't nominate two students, the school would probably interview a few of the top students and ask them where they would like to study to make sure that the person that they choose indeed ends up going to unsw.
It's all very complicated actually, but good luck for all you guys!