Hello Mr. Bob,
Those are a lot of areas to cover in 8 weeks. I'd consider pruning it down a bit so as to get a better look at chosen places and maybe enjoy a less frenetic trip.
For what it's worth, there are many worthwhile hiking (or tramping as they call it) trails or routes of varying difficulty in New Zealand, so if you decided to make NZ a destination, then bear in mind that the seasons are exactly opposite to what you get in the northern hemisphere. Probably the warmest weather months as far as can be predicted, are Jan, Feb, March.
If your journey is more 'sightseeing' than actual hiking then maybe warmer weather is not so important. Possibly actual scenic views in many areas of NZ can be a bit more spectacular in the colder months, (provided they are not clouded in!) as snow cover on ranges will be lower and more prevalent and that makes for some nice views. NZ being a relatively narrow strip and the only appreciable land mass in a vast ocean, the weather can be rapidly changeable, so if you're on a short visit then regardless of time of year, keep fingers crossed for good weather!
Good luck on your travels.