Yes, when you collect your hire car from the car rental company in Australia, it will have a full tank of fuel. Unless you have made prior arrangements with the car rental company, you will also be required to return the vehicle with a full tank.
You can arrange with the car rental company to pre-purchase the tank of fuel. Depending on the supplier, you can choose whether to purchase a full tank of fuel. You will pay the extra charge at the car rental counter.