There's a huge slew of reasons for this, but it's a sum of very high rents and wages, a small market and a general lack of competition. Ebay, Aliexpress and Amazon US are consequently very popular, but most retailers are international so shipping is slow and, for anything bulkier, quite expensive. (Also, the deals aren't quite as good as they used to be since the Aussie dollar has depreciated against the USD.)
So, if Amazon can build a local operation here that -- for the first time in Australia -- has the trifecta of low prices, wide selection and fast shipping, they will crush the competition, online and offline. Which is why local retailers are scared shitless.