Check with your bank first who imposed this fee - possibly it is a foreign transaction fee (imposed by your bank) as technically your tickets were sold in SIN (even if you are on AU website when you book an itinerary not from AU, you will be redirected to Amadeus booking engine in the country of commencement of travel).

Another warning - you will have to show the CC used to purchase the ticket at check-in in SIN - no exceptions - you will be denied boarding if you are unable to show the card.