I'm fairly certain (and the ATM list in this thread supports) that there is no Westpac ATM landside at SYD International. Both the train (AirportLink stations, unlike Cityrail stations, have no minimum purchase and accept American Express) and taxis (no minimum, but a 10-11% credit card fee will be added) will, however, take credit cards, which may be a better option.

A taxi may be your best bet between terminals, just be sure to let the traffic officers know you have a short fare, as the taxi drivers can get a bit testy about waiting three hours in the queue to drive 3 km.