Samsung Galaxy S6 G920I 4G 128GB Gold for $707.65. Shipping to metro Adelaide was just short of $20 but I assume will depend on your location.
I believe it is the Australian model not the International model Galaxy S6 G920I versus Galaxy S6 G920F but I don't follow this sort of thing closely so correct me if I am wrong.
Edit / clarification: I believe this is a grey import / parallel import of the Australian model. But from the Yatango link there is also a difference between the frequency bands available on the Australian model (G920i) versus the International model (G920f).
Be aware of Topbuy's reputation and Topbuy's reputation on Ozbargain as well.
Google cache says that it has been this price since at least 2nd Nov.
Also:
Samsung Galaxy S6 G920I 4G 64GB Black Unlocked Mobile Phone $708.05. Shipping to metro Adelaide was just short of $35, I have no idea why one costs double the other for postage I guess popularity gets factored in or something. Google Cache says it was $864.55 on 6th Sept.
It says it's an international product under the warranty section.