If you really need to collect some KKs at SYD, then why not change to a domestic SYD-MEL flight departing from the domestic terminal. Then arrive into SYD as an international arrival, clear customs as usual and re-check bags at the domestic transfer desk. Before boarding the multi-seated large capacity people conveyance vehicle (a long way of avoiding using the B word) to the domestic terminal, head to the KK outlet and purchase said contraband.
You will not be able to exit at SYD and rejoin the connecting international flight unless you have a D on your boarding pass. How do the customs folks know that you are not taking the full DF allowance into SYD, handing to a friend, heading to MEL and again entering Australia will a full DF allowance. That would be quite a rort and will not be permitted.