Good idea!
I gave it a go, but the seat selector only shows 57E / F available.
I rang Air NZ and the agent said there's currently ~40 economy seats free on the first leg and ~30 on the second so I guess they block out potential Sky Couch seats until close to departure.
He suggested asking for Twin Seat at check in and pointed out that "you can upgrade to Sky Couch at check in".
So that might be the best bet!