main round is where most of the the offers are made
after that is the late round of offers, where if there are vacancies, the UAI tends to go up, since there is more demand for the limited number of places left
only if there are enough vacancies and you put that preference up at the top of your list.... if someone else has their proference for that course higher than you... then they will get the spot and you might not get in.
2nd round offers are only for those courses that have vacancies left or have people droppin out from their main round offers. The chance of you getting a HECS 2nd or last round offer is close to nothing. They give priority to those who wish to pay Full-fee to get into the course, and also international students who represent big $$$ for the uni's.