From my understanding of GMO's, they can be crossbred with normal plants with pollen. So for example if someone were to bring GMO seeds to Australia and plant them, GMO seeds can be spread unto other farms via pollen, wind gust, etc unknowingly.
If GMO's aren't popular in Australia then just give it time. Farmers deal with overly competitive markets and need an edge... The edge being GMO's which produce exponentially better yields.