How will they measure RUC for EV's? Buy mileage at the Post Office? How would they measure the electrical mileage on a hybrid? That's probably the key and more complex question
EV RUC will be done exactly the same way as light diesels do now, buy it either online or in person at NZTA agents (Which include Post offices, AA, VTNZ, VINZ etc)
You purchase RUC in advance, and it is checked when you get a WOF, or can be checked by the police if they stop you...
As for RUC on PHEVS, while it is an interesting question, I think they will probably just whack RUC on the whole mileage and set up a complex way for you to apply to get a refund of the mileage you do under petrol power.( No don't ask me how it will work, it wont be pretty)
Currently you can claim back RUC for offroad driving - I am not sure many 4WD owners do - (you have to be able to show evidence,) but I think they will just add "kms driven under petrol) to that system