Essentially, your sell order will only be taken by buyers paying with the AUD in their wallet. There is some chance that a bid on another market (e.g., BTC/USD) is so high that Mt. Gox will do the conversion and buy your BTC/AUD offer and they would pay in AUD even though the buyer had a different currency. Arbitrage is what keeps the various markets relatively equal in price. (after considering the AUD/USD.)
Are you sure Stephen? I'm pretty sure MtGox is never a counter-party to any trade on their platform and that the individual markets are quite separate. Bitcoincharts seems to confirm this.
http://bitcoincharts.com/markets/