I've flown the
Mt Cook ATR-72-500s quite a bit in the last few years, they are fine. True, they aren't as cool as the old Avro 748s but it is an imperfect universe!
NZ isn't very big so you don't really need the 'near-jet' speeds of the Q400. I suspect the operating costs of the ATR may have some bearing on the matter too. We shall see.