There appears to be four places where currency conversion could take place:
1. WHMCS V4 itself if you set up multiple currencies. Say I have USD and Australian dollars set up. I need to choose one as the base. The client can decide whether to view prices in USD or AUD, and WHMCS does the conversion internally.
2. In the 2CO gateway in WHMCS as Fearz said above. (Thanks, I forgot that was there).
3. In my 2CO account. There is a default currency set there.
4. At 2CO checkout. 2CO checks the buyer's IP and translates into local currency.
Trouble is, every time there is a conversion process, prices change.
I did speak to 2CO live support last night, and they were helpful. Seems I need to set up a banking profile in 2CO making Aussie dollars the default. I can't do that until they finish their payment runs though, so I'll report back tomorrow!
Thanks for your support.
Cheers, Frank