those are promo fares & probably 1 or 2 only per flight.
These aren't promo fares, they are the current lead-in fares in (P) class. New Zealand Domestic fares are cheaper than Australia.
They could get Alliance to operate them, not NZ.
The flights aren't operated by NZ already, they are operated by Mount Cook Airline (72-600/500) and Air Nelson (Q300). Both of these two airlines already have an lower cost base that NZ it self.
Why would they want to get Alliance to operate the flights with there 30 year old ex Ansett F50s? When they have there own fleet of cheaper to run 72-600 which are less than an couple of years old?
Using Alliance would barely achive anything apart from customer confusion and lower customer standards.