Oh yes he has! This is no different than a bank giving (for whatever reason) 10% interest on its accounts. 6 months later, it decides that since everyone else gives 5%, it should give out 5% as well, and apply this interest retroactively (by cutting any non-witdrawn interest in half). No problem changing the rules going forward (you can always change bank), but dinging the unclaimed interest is robbery.

I hope that Australian consumer protection laws and its judiciary system are strong enough to punish this shenanigan.