Fare of country of purchase vs fare of country of departure - the higher of the 2 will apply. The exception as detailed in the rules is Canada where the price of the country of departure will apply. You have to buy an ex-SYD xONE4 in either Australia or Canada to get the Australian fare which is lower than the US fare; buying an ex-SYD itinerary in the US will result in the higher US fare.

The base fare is identical whichever carrier you buy your ticket from.