What do people usually wear for postgraduate studies in Australia?

Why don't you contact the school or go online and check out their information. If there is no indicator of what to wear, I'd say that's a safe assumption that there is no dress code and you can wear what you like. But, be warned, Australia is not as casual as the USA, I've lived in both places, so while it's not necessary to wear suits or blazers, more business casual than grunge casual would be appropriate. When all is said and done, don't stress it! It's school, not a job interview - have fun!

Eight years a student, twenty years a lecturer in Oz:

There are some differences between Departments so the Business Studies post grads might be more formal, but campus wide there are usually a full range of styles from jeans and tee-shirts to 'shirt and tie' outfits, although most students are at the 'cheap and casual' end of the spectrum.

It gets cold in winter - July to September- so be prepared to wear a sweater, even a jacket. Clothes are pretty inexpensive - so it's not going to be too difficult. Hope you have a good time here.