On flights from Sydney to JB via Perth you must also be prepared for rifles to go through customs in PERTH as the Syd to Perth flight is a Domestic flight (often there is only two hours between these flights) .
To further reduce weight I also buy my Toiletry's, Battery's, Drink etc in the last Airport before the flight.
This is often at Jo'Burg Airport on the way to Zim etc. Jo'Burg international has quite a good Duty Free section.
Most of the time you will have a few kgs tolerance before the charge, but not always.
NOTE; At Perth you WILL have your hand luggage weighed (by Nazis) and if over weight you will be told to check in the excess.