Before visiting Australia, it may be wise to exchange a certain amount of currency before you arrive here, than wait until the last minute at the airport.
When in Australia, you may find it useful to exchange at banks or at Post Offices, where you will get a rate more close to the international exchange rate, usually without a commission charge.
ATM withdrawals can be convenient, however you will find it difficult to know the current exchange rates to compare. They also usually charge an ATM fee on top of possible unfavourable exchange rates.
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