You cannot get Aus$$ here with no commission - the so called 'no commission' deals have typically a 5% commission built into the exchange rate. OK some are a bit less, some maybe a little more, but all the outlets have it.
Get them in Australia from an ATM using your Nationwide card. No commission, no exchange rate loading.
Don't know about changing money any other way in Australia - the banks there have a reputation for high charges but I don't know how far this is true or just travellers' hype.