Give Shell or Mobil a call, and if they don't supply it locally, they'll probably know who does. Then ring for the identified supplier's retail outlet. Or look in the YP's for avgas refuellers, such as there will be at Jandakot. Or Barbagallo racetrack's office might be able to give you a heads up.
Bet you'll need a tip truck load of money for a tankfull.