I personally use kobo and books on board.
http://www.kobobooks.com
http://www.booksonboard.com
For books on board the best way only view books available in Australia is to register and set your country to Australia. When logged in it will then filter out books that are unavailable in Australia however if not logged in it will display all books that are available on the site (i.e. it also has the geographically restricted books).
I tend to use booksonboard to suss out books that may be available , log in see if they are if not then go look at kobobooks. This is because I think the navigation on kobobooks not necessarily the best. Often booksonboard is also cheaper than kobobooks however this is not always the case.
Also I personally went with a device that supported EPUB books, and Adobe DRM as I felt this would provide the most variety in stores to buy from. The Kindle somewhat locks you to Amazon (not completely but somewhat).