The big issue NZ Bus will face if they move the vehicles to Auckland is that they will not currently receive a higher subsidy from AT for using these vehicles. The GWRC provide a higher subsidy rate (and additional incentives) for tenders pertaining to the existing trolley bus service and presumably, the new electric bus service. The only other council that provides similar subsidies is the Waikato Regional Council who require CNG buses to be used on some urban routes.
Charging infrastructure will also be a greater issue as routes in Auckland are generally longer than those in Wellington.
Still, it's great to see some positive actions being undertaken to move away from diesel buses.