(The inbound tourists don't have a choice. Most of them arrive on tours that cover the country in 1 week. And they want to see Rotorua and Queenstown as part of their trip. There is no alterative to flying. To drive that route would take at least 2-3 days. I'm sure they would prefer a jet, however Air NZ is the only game in town. Even if Pac Blue decided to go after that route with a E-jet, the question is whether you have enough volume to meet your 61% breakeven point. (60 seats in a E-190) While competing against an Air NZ ATR or Q400.)
Qantas does fly Queenstown-Christchurch-Rotorua with 737s, so AirNZ is not the only game in town. ATRs or Q400s might give better frequency. OriginPacific had ATRs to fly this route for Qantas.
Break even fares are a bit hidden, because if you are talking about a tour group, Qantas or AirNZ will have sold a bulk fare to the wholesaler. Do many NZers fly between Rotorua and CHC and ZQN anyway?
I think the Q400 has a lot to offer NZ on some routes. If AirNZ does go for the 90 seater Q400X, and decides to use it on long sectors like AKL to the far south of the South Island, this country will be down to AKL-WLG and AKL-CHC and the tourist route Rotorua-CHC-ZQN as the only routes that use jets at all. Even AKL-WLG, a Q400X could give half-hourly departures, and no difference in block time, but would people accept this on the biggest domestic route?