For a guaranteed extra seat, ask Air New Zealand's check-in counter staff on the availability for your flight. See the tables below for Twin Seat price details, which are per person per sector.
Examples:
- From NZ: domestic, or to Pacific Islands / Australian east coast ports - NZ$29
From NZ: to Perth, HNL, Japan or west coast USA - NZ$99
From East coast Australia: to NZ / Norfolk Island / Rarotonga - AUD$29
- From Perth: to AKL/CHC - AUD$99
more at the link: https://www.airnewzealand.com.au/buy-a-twin-seat-at-check-in
But if the seat was empty next to you, then you could've commandeered it for free…