Just created another section to the links: bookshops. Down the bottom right of the page.
Bookfinder and Addall search various book seller sites - Amazon, Alibris, Abebooks, Barnes & Noble, etc. They also factor in exchange rates and shipping prices - so make sure you set "shipping destination" to "Australia", and "currency" to AUD. http://www.bookfinder.com/; http://www.addall.com/. Better World Books raise funds for third-world literacy, have quite good prices, and very cheap postage - $2 worldwide! I don't know how they do it! http://www.betterworld.com/
Then of course there's the local Christian book sellers: Koorong, Reformers, Moore, Word and Christian Books Australia.
Koorong is big and (usually) cheap - but wait for sales, and always check that you can't get the same price (sometimes cheaper!) at Reformers or Moore. Always check Koorong's markdowns - I've picked up quite a few unexpected bargains there. Quality theology doesn't sell well - a pity for everyone else, bonus for us! www.koorong.com.au
Speaking of of quality theology - Moore and Reformers have an excellent selection thereof. Moore has a broader range; Reformers is reliably Calvinist-Reformed (hence the name...). www.moorebooks.com.au; www.reformers.com.au. Word often has unexpected bargains - keep an eye on their catalogue. If you're in town and have a few minutes to spare, their city store is worth a visit. www.word.com.au
Christian Books Australia is the old Rockdale Christian Books. They have an outstanding selection of second-hand Christian books, both theological and devotional. Their shop is in Rockdale; but if you're in the southern highlands, drop into their book barn. I haven't yet - dunno if it's a good idea - I'll probably try to buy the whole barn... http://www.christianbooksaustralia.com/.