Scott mentions that tickets can only be purchased for some of the last minute fares but it also leads into the regular booking workflow at Air New Zealand which can lead to ever lower priced tickets.
This looks like a great option for a flexible traveler in NZ especially if you need cheap one-ways. Most of us will book award flights directly to the NZ destination (i.e. Queenstown)
but if you run out of allowable stopovers this seems to be the way to go.