It should be easy enough. Most machines will not charge you anything but watch out for privately run ones that charge a small fee. Be aware of personal safety - machines in some areas are watched over by a bank guard during working hours so maybe best to use them then. Also a few machines dispense $US - handy if you want them but not otherwise.
Nationwide will charge you 0.84% (or it might be 1% now) which is considerably less than any other method of exchange.