It's not really a Sony thing. Regardless of device, you'll want to have a US address and most likely a VPN also if you want to get around geo-restrictions at most online bookstores. I won't post instructions for using a VPN because it differs depending on the VPN service you use, but it's generally fairly straightforward, albeit a bit cumbersome too.
DarkRoast, it's generally a pain in the backend, purchasing ebooks from Australia. The people here have already given you some great resources, but if you have any specific questions, feel free to ask them too. Two suggestions I can make right now:
1)
Search for a nearby library on Overdrive. See if there are any libraries nearby that have an ebook collection. It's possible with many of these libraries to create a membership even if you don't live inside their council boundaries, as long as you can actually go to a branch.
2) Try
KoboBooks. Seriously. I think they might actually share much of their catalogue with Borders AU, and if you subscribe to their newsletter, they regularly send out nice coupons, such as a 50% off one recently. Their selection and pricing of
local ebook content by I believe is even better than Amazon's Kindle Store, which is commonly said to have the largest range and cheapest prices of ebooks overall. Either way, this might not matter much to you depending on your reading interests.
Unfortunately, with many online bookstores, the prices are outrageous if you're looking at books by Australian authors. For some reason, booksellers / local publishers seem to think still that $20-32 AUD is a reasonable price for a new release ebook. (My response is merely 'screw 'em' until they're no longer new release. It was funny watching Howard's 'Lazarus Rising' drop down from $34.95 to $19.95 AUD within a few weeks.) Ebooks by overseas authors are not always, but often enough, cheaper and in the price range of $5-16 AUD. The former situation is possibly why you're seeing inflated prices at Borders AU and A&R.