Often USD has the lowest rate of screwage (word?). So much so that it can be better to change local currency into USD prior to departure, then change the USD into AUD on arrival in Aus.
Yes, you get screwed twice, but that can still work out significantly better than converting less common currencies into AUD directly.
Interestingly, if you look at Forex trading web sites, the same applies - swaps between USD and any other currency usually offer better buy/sell margins compared to swaps between 2 other currencies.
Alternatively you keep the USD in a shoe box for when you take a foreign holiday somewhere, and then use the cash for the holiday (probably get a good rate for your USD on arrival at your holiday destination.)
The Euro could be similarly liquid, I haven't checked recently.
Edit: agree with Ken here:
I found that once a currency leaves its natural domicile it becomes, in a perverse way, more valuable when you need to acquire some
That pretty much agrees with what I'm saying above... Get rid of your local currency before you leave, they want it, but Oz exchangers don't. However every currency exchanger has a safe full of USD, so they can give you a much better rate on USD.