If accommodation is what you're worried about, obviously check when you're going to be charged first. If you're not charged until you've moved in, then I can't see why you shouldn't apply.
I'm not exactly sure how living on campus works. But if you have to pay a small application fee, it may be a small sacrifice. The larger the initial expense required, the more you may want to hold off.
In saying that: Seeing as though you're aiming to get into commerce, you can also consider picking UNSW anyway, but starting with a related degree. Like say, B Economics. Then get yourself adequately high marks to transfer into commerce later on. (Plus, in degrees such as the BEc, you can take units that get transferred over into the BCom.) In short, there's also alternate pathways into UNSW commerce.
(It's not fair for me to just tell you what to do specifically. It's ultimately your own decision or gut instinct.)