Thanks Moondog.

In my case I don't need to look up the interbank rate because my receipt explicitly says the markup was 3.75%:

"This transaction is based on Reuters Wholesale Interbank exchange rate plus 3.75 percent international conversion margin."

So Virgin Blue did disclose the markup after I completed my transaction, but not during. Nowhere during the booking process was I informed that I was going to be charged in my own currency at a mark-up, let alone given the option to decline it. I even went through a dummy booking process to double check.

I'm actually a little surprised that Virgin Blue is so brazenly violating Visa's rules.