Preferences don't work like that. You're offered the highest preference you qualify for and that's it. If you refuse to accept that offer in the main round, you have to switch your preferences so the one you wanted is higher and hope the course wasn't filled in the main round.
agree that it's all on UAC site. The main round is the one in which you generally get an offer.
just as a side note - to my knowledge the general advice given if you want to switch preferences after main round is that you accept your main round offer, then change your preferences, then hope you may get a later offer for your new preference. This way, if you don't get an offer in a later round (nothing guaranteed, even if you were over the ATAR requirement, because course may have been filled), you still have an offer (the main round one).
Why is there a round before I get my ATAR?
some universities offer places in the early round (before ATAR's come out). These are normally based on other factors than ATAR eg. distance, course, specific circs. Some uni's (eg. UNE) have specific early round entry requirements that you can go for (basically, get school to recommend you or something). But most people just get offers in the 'main' round.
