I think the difference between Air NZ and Qantas/JetStar is probably willingness to incur extra costs. Air NZ is changing its routes out of the South Island, and flying under the ash, both of which burn extra fuel (I think a spokesperson said something about 10% extra). Qantas and JetStar obviously have more aggressive accountants, and also, of course, NZ is peripheral to their operations.
You know you're old when you can remember when Qantas was a really good airline.