Hi Natalie,
The three australian ebook stores I know of are Dymocks,
www.macmillandigital.com.au and
www.harpercollinsebooks.com.au.
Of those three harpercollins is the more competitve with a 20% discount on all ebooks and some ebooks are cheaper in AU$ than the US price on BoB or Sony store. I have bought a couple from them.
macmillan digital has 10% discount and as you know Dymocks have no discount.
The main problem with Oz stores at the moment is the limited range and they only sell PDF for Sony readers.