Are you sure that all passengers going through the "Australian Passport" line hold Australian Passports?
I was invited to "tea and biscuits" with the Manager of Brisbane Airport Customs when I queried, in writing, why other than Australian passport holders were being processed in the "Australian Passport" line. After saying that my delay on one occasion caused me to write to him... too right!... he admitted that he didn't have the staff to sort out the queues and that it was going to get worse. I honoured my promise not to tell anyone for a few years... he asked me to!!!