It is all automated; when you no-show at SYD for SYD-BNE the computer will cancel BNE-LAX, release the award inventory (which is then not returned to awards for same day travel). So when you try to check-in at BNE there is no reservation and no inventory to rebook it. Nothing that the agents can do (except to force conversion of paid seat to award, and it takes a high level supervisor to do that -- unlikely to be done except as a very big favour). The AUD70 change fee is cheap (it is USD125 for OWE tickets, for example).