Also, may be the merchant is a bit doggy and charged you card in AUD,
with their doggy conversion rates ! Sometimes they have this rip off practise in tourist spots, paypal also !
I have only ever had this happen to me at Heathrow - where the merchant always offers to charge in AUD (and it happens there all the time). I have always said no to this, but a couple of weeks ago, the merchant just charged it in AUD, without asking me. I had a bit of a whinge (over-whinge, to be honest) and made them refund the original transcation, and charge it again in GBP.
Out of interest, all 3 transaction show up on my statement, and the difference was 3%. It was only a small purchase, so it wasn't worth the whinge really, but still, good for future reference.....