For all the popular subjects (unless its tiny but probably even then) copies of the textbook are available in the library. I think it might even be some kind of rule. It really depends on the subject, for some, it'll be really handy to have the textbook around your house. For others, it's probably something you'll need to read every so often. I think for first year science subjects it is generally rather useful to have a textbook around your house (even if its a version old, it hardly matters)
Whatever you do though, don't buy them from the university bookshop. I made this mistake in my first year. Yes, it's extremely convenient, you can get all your books in one hit and be done with it but you are paying for that convenience.
I recommend to checkout
http://www.booko.com.au if you want them new. It looks at maybe 30 different bookstore prices and shows you the cheapest one (includes book depository too, so, theres no sense in going direct to there, booko might show an even cheaper place).
If you want secondhand, google textsbooksexchange (cant be stuffed finding the website) or use monash marketplace.