I suspect the reason Australia misses out for merchandise on the official SE store is simply shipping costs and especially the Australian government's high import tarrifs for imports on top of the GST (not surprising the SE online game store for Australia is entirelly online/digital download and that naturally skips those extra charges and only has GST).

Given the Brexit debacle the Australian government is still in trade negotations with the UK, this is probably something that is not going to change any time soon unfortunately.

There are third-party stores online that sell FFXIV merchandise, but their prices can be rather dear unfortunately.