If you're looking to book tickets online and you're using an international card, follow these steps to ensure you're entering the correct billing information for your booking.
During the purchase process, at the ticket delivery method selection screen, you will be prompted to choose whether your billing address is in "Australia", or "Other Country". If your billing address (that is, the address under which your card is registered with your financial institution) is international, select "Other Country".
At the next screen - the payment screen - you will then be able to choose a country other than Australia from a drop-down box, allowing you to input the billing address of an international card.