As the others have suggested - the posted exchange rate that you see on TV and in the paper, is just the mid-rate. In other words, if you were buying US$ with AU$, or AU$ with US$, your rate of exchange will be a couple percent on either side (to cover profit of the exchange service... or if they charge an actual fee, it covers them if they need to convert their own currency stockpile before the rate changes too much against them).

So it might say US90c, but to buy US currency it you will only get about US88-87c.
And in reverse, it would take about US92-93c to get back an AU$1.