The de facto visa also gives you the right to live, work and study in Australia for two years, after which if the de facto relationship is still stable, exclusive, etc. you're then eligible for permanent residency.

Well damn. That's inconvenient. How about "still stable, exclusive and any time we have sex with others it is because we are Bad People and cheating"? ie. It would be a shame if polyamorous people in stable primary relationships were penalized for using different moral vocabulary.