Secondly, you can't do this for NZ - the Amex voucher is for BA operated flights, and they only go to Oz (or Singapore/Bangkok/Hong Kong from where you can connect).
It is also worth noting that the AMEX 2-4-1 won't be valid to Melbourne when they decide to change that flight to a BA codeshare. As discussed elsewhere it could end up that SYD goes this way too.
