It should really be seen as supplementary to your efforts of trying to get into Uni, not a replacement.
Selection, from what I understand, is only partially equity based, with aptitude, and the UAI rank still considerable factors for the selection of entrants. The applicants will most likely have been disadvantaged in some way, shape or form in their preparation and completion of the HSC but can prove their ability to perform had it not been for those factors.
While the criteria itself is very vague, coming from an upper north shore school and simply being too lazy to perform is not going to be a valid excuse for acceptance (at least it shouldn't be). [as an example of what factors you shouldn't be applying for Unitest]