Buying forgein currency on board

I see P & O mention that you can purchase currency on board. Has anyone done that and what are their rates like compared to Travelex etc.

Doing world cruise sector and have US $, but not New Zealand, Australia and Hong Kong currency.

Is it better to order before you go or on board?

We usually get our currency onboard, rates are usually reasonable, it's convenient, it goes on your account & they take it back if you don't need it.

Right thats what we will do then as much more convenient.