Booklists should be released when it gets closer to the start of semester. Some degrees might have already released their booklists on their faculty websites. Otherwise, you could check here --> https://www.bookshop.unsw.edu.au/textlist.html
In terms of buying textbooks, I think it depends on your faculty. Facebook has many UNSW Textbook Exchange Groups, but not for all degrees and faculties. The common ones I see are mostly law, commerce and engineering. Alternatively you can buy second hand textbooks from Gumtree (might be hard to find) or StudentVIP. Zookal is also a good place for cheap new textbooks. You could also try searching up your [Textbook name] or [ISBN] and add PDF to the end of your search on google if see if anything pops up [if you're looking for a soft copy].
Also, I think its best if you wait until your first lecture before you buy any textbooks. Sometimes, the materials you get in your classes are sufficient enough to get a good mark, or you could hit up older year students who may be selling them on campus. There's also a UNSW Bookshop on campus and a Library if you are just looking to loan.
If your doing STEM don't bother with textbooks because firstly you won't need them and secondly there's pirated copies floating around in Unsw that you'll be able to find on the rare occasion you want them...
The prescribed course material provided by your different courses would be sufficient in most cases. You may wish to order these from the bookshop so this will be an expense. Buying textbooks can be tricky. You may end up wasting both time and money. Best thing is to wait until your first lecture.