I won't be much help because I have never done an international return.    But I'm pretty sure you cannot generate a return label.    How could you, Australia Post does not operate overseas.   I think you have to negotiate a price for postage with the seller and offer to pay them that amount.   So the first thing I would do is email your buyer and explain you cannot generate a return label because you are in Australia and it would not work, and ask her to obtain a postage quote.

What concerns me though is why not fight the return?    You said it is authentic.

Or worse, you might have to ask whether it is worth it.    You will lose the postage amount you used to send the item and the cost of the return postage.   Maybe $48?     I would be looking at how much the dress sold for.

Also I believe it is correct never to refund until you have the item back.

Hopefully somebody with more experience in international returns will be able to help you soon.