The lack of GST has nothing- I repeat, NOTHING to do with the reasons stuff from overseas is cheaper. The real reasons are a combination of “Economies of Scale” (buying stuff from places like the US with larger population bases), “Higher costs (rent, wages, etc) to operate in Australia”, and “Greedy Retailers” with, IMHO, an emphasis on the latter.
GST is only 10%. A book purchased from somewhere like The Book Depository in the UK is quite likely to be 50% cheaper than the same book from one of the major retailers here. Even if GST was introduced on online purchases (and I shall be vigorously opposing such a move), the fact is that it’s still going to be vastly cheaper to buy stuff from overseas than it is to get it here.