I'm pretty sure Amazon is capable of calculating different taxes correctly for various states in the US so I don't think calculating GST for Australian shipping addresses is really going to be that challenging for them. I suspect they just want to force every Australian onto the local portal.
I'm more concerned about the per package fee proposed by the Home Affairs Department on parcels. This will mean my $1.20 widget including postage (plus GST of .12, {will they also charge GST on the shipping cost portion?}) from China will now attract an additional $5 Home Affairs handling fee. How is it going to be collected? Why doesn't the Government just add the 10% GST charge at the same point? Given we have tax treaties with a lot of countries, will any overseas VAT paid be credited against GST to be paid locally? If not isn't that double taxing?