From
https://www.airnewzealand.co.nz/spec...ildren-infants
Unfortunately, you can't book an infant fare until your baby is born. You can book everyone else's ticket, of course, and we can add your baby after the big day. Simply get in touch with us and have your booking reference ready.
When you're flying internationally, infant fares are 10% of the adult fare.
Have done this before for domestic flights before by calling the call centre and was fine.