Nothing pisses me off more than another kiwi telling a foriegner that there is nothing to see in the North Island. Complete BS. What a completely stupid and ignorant comment. Sounds more like something an aussie would say.
For some excellent flying in the central north island go to Taupo airport and take a jolly through the Kiamanawas and up and over Ruapehu (an active 10000ft volcano). No volcano's in the SI. Additionally, in the North you will be able to train with some guys who have a massive amount of local and international experience, not just 10000hrs of instructing. There is a bloke called Bruce (?) who runs a school out of Taupo airport. He took over from Peter Avery in about 1995. This guy has 17000hrs of work in all manner of american and russian gear all over Australasia and PNG. He can show you how to fly a R22.
Living costs are cheap in the North Island and most of the operators will help you out with accomodation.
Have fun.