Don’t forget, too, the issue of market size, population, and isolation. Admittedly, this shouldn’t be an issue with e-books, but for physical goods it makes a difference.
Australia is about 30 times the size of the United Kingdom, but has barely one-third the population of the UK. Australia is the same size as the continental United States, with only about 6% of the US population. I live in Southern California, and within about a three-hour drive of my house is a population equal to that of the whole of Australia.
All of this stuff is significant when it comes to setting up something like an Amazon warehouse. The size and wealth of the market here in the United States means that it is far easier, in terms of economics, for a company like Amazon to set up huge distribution centers with tens or hundreds of thousands of products. Economies of scale allow Amazon in America to stock such a wide variety of items because the sheer size of the market means that there is some demand even for relatively unpopular items. The same economies of scale also allow Amazon to ship all over the United States from a relatively small number of large distribution centers.
Setting up something similar in Australia would be far more difficult, because the relatively small size of the market would mean disproportionate overhead for any company that wanted to supply the range of goods that Amazon provides. If a company like Amazon were to set up shop in Australia, you can be sure that the variety of items in stock would be far below what the company can maintain in the United States or the UK. It would be much more likely to focus on a narrower range of particularly popular and/or profitable items. And none of this even takes into account the costs, for some items, of getting the merchandise to Australia in the first place.
I’m not arguing that the high prices for books in Australia are necessarily justified. Even when i lived there, over a decade ago, i was always conscious of how much more i paid for books in Australia than buyers in the US were paying. I’m sure that the “cozy monopoly” mentioned by Giles is also a factor here. But even if all of this were eliminated, Australia’s size and population mean that it would likely not benefit from economies of scale in the way that the US or even the UK do.