It should not be a currency issue since the AUD and USD are trading at about even.

My best guess is that the agent who did the transaction took the amount from the wrong line, in other words, charged the total fare but did not have the taxes/fees/surcharges included.

How this would resolve itself, I am not sure, but since UA said the record shows paid in full, I would say that you just got a ~20% discount off the fares.