As the Reg says, it's not the GST
A few months ago I bought a box set of the first four seasons of Doctor Who as a Christmas present for someone. Here in Australia the best price was at JB HiFi at about $287. In the end I bought it from Amazon UK for $64 with free shipping.
There is no way that adding 10% GST on to it would have changed my mind in purchasing from amazon UK!
I've also bought the BluRay Boxset of the complete Harry Potter films for $49 (free shipping again), in JB HiFi it's for sale for $139 + postage. Oh, I also got the BluRay edition of the extended Lord of the Rings trilogy but I've forgotten how much I didn't spend buying that here :-).
Prices and exchange rates have changed since these purchases but there is no way that a 10% GST explains the price difference. Just look at that Doctor Who boxed set S1-4, $287 vs $64, I mean WTF! Someone, somewhere is gouging...