You don't withdraw in AUD from international platforms, instead you trade back to a currency supported by btcmarkets.
I.e Buy Etherium (or LTC, BCH, XRP etc) on Btcmarkets, transfer it to one of the big exchanges (binance, bittrex etc), trade whatever altcoins you want at low fees. Then when you want to cash out sell the Altcoins into ETH and transfer back to Btcmarkets to sell.
