You can buy textbooks at any time and there's several options available. Your lecturers will generally tell you when they want you to have the texts by, but in my experience it's usually within the first three weeks. If you're looking to buy them, here's how you can go about it:
A) You can go to the United Campus Bookshop near the Student Union building. They are always up to date, so you know you'll be getting the edition you need. However, you can expect to pay up to $600 for a semeseter's worth of text books.
B) You can go to the Second-Hand Bookstore, also located in the Student Union building. As the name implies, these are pre-loved texts, and there are no guarantees that they'll have the edition you need ... or even have the book in stock. It's naturally cheaper, but not by all that much (you'll pay about 60-75% of the UCB prices)
C) Returning students are always looking to sell texts they don't need at the beginning of the semester. It's much faster than the Second-Hand Bookstore because when we go through them, we don't get paid anything until the books are sold. We usually post book lists early and you might be able to barter us down a little. Be quick, though.
D) You can order your books online. I've never done this, but I daresay you'll get it at a much cheaper rate than other methods. I don't know how long they take to ship, though, so you may need to organise to have them sent to your college.
E) Your courses often have required texts, but several of them also have recommended readings. These are often supplements and are stand-alone. You won't need them to get through the unit, but if you're looking at buying them, I suggest going halves with another student in the same course, or, in the event of more than one recommended text for the subject, you buyong one and someone else buying the other and simply swapping when you need to.
Your best bet is probably to go with options C and E (if it applies). It's quick and easy, and you're usually lined up to get current editions (or earlier editions that your lecturers will still accept).