You have to put them right and if they don't make good any shortfall then report them to the Fairwork Ombudsman.
Even if you are getting paid above the award, why not go to the union and see if they can get more money for you?
I know of at least one situation where the pilot was getting paid $13k over award and the union got more money out of his employer for him!
So - go right ahead, it can be done.