Carnet Cards are a bit confusing ParksFlyer.
I got BP and Shell direct (Shell carnet replaced by normal credit card now). Never needed a Mobil until, you guessed it, I was in Mildura WITHOUT a fuel man in his hut!
You rearly need a BP, Mobil, normal credit/debit card (with pin), and cash/cheque book if you are planning to travel. Skyfuel (there are probably others) should be able to help with the first two:1300 759 383 (PO Box 6006 The Oakes 2570.