Even better than just going to the Book Depository (which does have free postage - as does Betterworld Books), is to check out:
www.booko.com.au It's an Aussie search engine. You type in the name of the book or DVD and it automatically searches worldwide for the cheapest price (it automatically converts it to Aussie dollars and includes postage, if any). Brilliant!
Oh, and there's also second hand books to check out at Betterworld Books.