Aye, they did make a change in March. So, if you go with AUD, you will get hit by a 3% or 3.5% charge, depending on whether it is the Visa/MC or Amex. It is also likely that you will get hit with a poorer exchange rate if you let the merchant do the conversion. In these situations, you are nearly always better off being charges in the local currency.
Thank you to both of you. Just did the charge in SGD on the Westpac for the SQ flights (loving the triple bonus offer). I then used my new 28D card to pay for other flights on FlyDubai and it's already showing in pending transactions and it comes up a few dollars cheaper then what I calculated on xe.com - not bad.