In Australia is it illegal to hold back commission?

Our company has just introduced a policy where up to 15% of earned commission of the entire sales team can be withheld if our aggregate Mystery Shopper score for the month is not over 80%. We believe this is unfair, as Mystery Shoppers are individual based assessments and have nothing to do with how an entire team is selling. For example if "John" scores 20% on his report, the entire team loses 15% of their commission for the month. Is it legal to dock another persons pay, based another person's assessment? Bear in mind that originally we used to receive up to a 20% bonus if we got achieved over 80% on our reports. This is effectively a 35% pay cut on our commission.