Just thinking of the build stages there will be, between that and what there is now!
IMO, a world class airport NEEDS to combine the international portion WITH the domestic terminals. New tourists to Auckland (that are connecting to domestic flights) are often lost upon arrival. A choice to either walk to the domestic airport or wait for the transfer bus (every 20 minutes) is not really acceptable.
This reminds me of what they did to YVR in Vancouver beginning in the early 90s. Totally awesome, by the way.