If you use the standard 'Market' order on the 'Trade' page, you need to enter the buy amount in BTC.
But what if you want to buy a specific amount in AUD, not in BTC?
Option #1: Use 'Express Buy'
Option #2: Use the 'Recurring Buy (DCA)' order type where you enter the amount in AUD and it instantly market buys it for you. If you really don't want the DCA order to run again periodically, don't forget to cancel it.