from my experience on holiday (South island only) all the main roads are sealed and much smoother than the UK, even a lot of the unsealed roads were smoother than some of the UK sealed ones. We were pleasantly surprised.
I suggest that you ask the guys on
http://www.kiwibiker.co.nz/ about riding all year round.. But as the climate is a lot milder - apart from the very south of SI then I would say you probably can unless you want to cross the alps often in winter. Here in Yorkshire I ride all year apart from about one month when there is snow and ice.
I am planning on taking my SV650SK3 out wityh me as a comuter bike and possibly depending on where we end up buying eventually an on/off road bike as I would love to explore some of the less travelled roads.