When I participate in swaps from Canada to the USA, I send a couple of US dollar bills (which I have leftover from vacations in the States) with my blocks, to pay for return postage. That probably wouldn't work for you to Australia, though.
There's something called an "International Postal Reply Coupon" that you can buy at your (US) post office and send with your blocks, and then in Australia (or wherever) whoever's hosting the swap can trade it in at their post office for Australian stamps to send your blocks back.
Jan