I use gandi.net for a lot of my domain registration. They allow payment in your choice of US Dollars ($15 per year), Euro (12) or UK pounds (8.30). So when today I had to renew a domain, I checked what the cheapest option in Australian dollars.

US $15 = A$17.38
EU €12 = A$20.05
UK £8.30 = A$19.38

So no contest, USD is the way to go. Thanks for providing the option, Gandi!

(Yeah I could switch to a cheaper registrar. But I don’t have the time or energy to do so at the moment, thanks for asking.)