I toured mostly on the south island (5 weeks) and a little in the north (10 days).
NZ couldn't be any more perfect for bike touring. You should go for as long as you can. i wished i had 4 months.
Try to find the guidebooks "pedaler's paradise" (I got my copy here:
http://peteandedbooks.com/ by calling them, a few years ago). Those books list nearly every rural road, with elevation profiles, distnaces, and what services exist at each town. the books also have suggested routes for different length stays, and info on how to best take advantage of the weather/seasons/wind direction.
There's also a lonely planet cycle touring book, which has good general info but the suggested routes are rather short, and you'll need additional maps to connect them. If you are a AAA member in the US, their auto club reciprocates membership. You could easily get by just with pedallers paradise and free maps from tourist bureaus.
In most areas the towns are spaced less than every 30 miles, although sometimes the towns are tiny. There are hostels/backpackers all over the place, tons of campgrounds ("holiday parks") that usually have indoor kitchens (bring cookware but maybe don't need a stove if you will use the holiday parks). many of the holiday parks also offer cabins or trailers, which would be good value for a couple. there are a couple of chains of hostel/backpackers, some people had membership cards that made them a bit cheaper, but i didn't use them much. 2004/5 prices ranged from $5-15 to camp, and from $15-20 for a bed in a dorm. Food prices were similar to the US. Do NOT miss the ice cream.
Most towns have at least a pub that will either rent you a room or sometimes let you camp out back. People are insanely friendly -- i wasn't free camping, but i bet it would be really easy. medium size towns have tourist information centers that have uh... lots of tourist information. There is internet all over the place. money changing is easy, just use your ATM or VISA most places.
The roads are narrow and hilly (steep!) and twisty, but not very heavily trafficed. north island is steeper than south, but both are very steep. Low gearing is highly recommended (MTB). The road surfaces are generally very clean chipseal, from smooth to irritatingly rough. Regular road-touring tires are fine. The wind is strong, and changes direction daily. It rains a lot on the west coast, so you might want fenders. don't plan on buying bike gear there, it's expensive.
favorites: wanaka, arthur/porter pass, crown range road, "the neck", dunedin, milford sound, hokey-pokey ice cream, the east cape of the north island (the inn at whangarea)
less fun: queenstown, aukland, christchurch, 5 days of continuous rain on the west coast, getting blown off the road by strong winds, sandflies, keas (parrots) that like to eat your bike seat.
have a fabulous tour!!! (I wanna go back!!!!)
