http://www.qantas.com.au/info/flying...Guide/sydIntT1
There are showers landside in the international terminal, however as mentioned on the QF site, it is bring your own soap and towels. So be sure to do that in your baggage, along with a waterproof bag for storing the wet towel afterwards. Alternatively the airline lounges do provide towels etc. if you have access to lounges (e.g. OW Sapphire can access QF domestic lounge).