The exchange rate also would be a factor, the Australian dollar has fallen below parity to the US dollar so a $320 set will cost about $350 before postage (approx $70).
Also, Lego has introduced free shipping for orders over $150 in Australia, so the $400 set will not cost any more for postage at the moment. There are not alot of retailers in Australia expected to carry the item, however there may be a bit of a change to that being the item is an Australian model and new retailers may be engaged to sell it.
I have been told directly that the actual Sydney Opera House will be selling the set, how many other Creator sets can claim this honour?