What do you mean to go back? Current rates are still shit. Current visa exchange rates are 1.00 USD to 1.56 AUD.
But to answer your question, it is in part because the interest rates in the U.S. are higher. This makes their bonds more attractive and therefore strengthens their currency.
This is in part is one of the reasons reserve bank has to keep putting up our interest rates so our currency doesn't get completely raped.