We had planned to spend half of our time on the South Island and the other half on the North One. Things worked out differently: We ended up spending 5 weeks alone on the South Island until the weather became too nasty. The weather in New Zealand was a very changeable thing: In one respect we were very lucky as we did not have much rain. Even on the West Coast, the so called Wet Coast with an average yearly rainfall of 8 meters (!) we just had one really rainy day - that was all.
Unfortunately, we also had one day with snow (which melted pretty quickly, but it was still some sort of traumatizing experience to wake up by the sound of snow falling onto the tent). In another respect we were incredibly unlucky: 80% of the time we had a head wind - sometimes so strong that we could not cycle any more out of fear to be blown off the bike. And whenever we changed direction, the wind changed direction, too. I guess you can't have all the luck. In the end it just got too cold which was aggrevated by high humidity i.e. a lot of dew in the morning everywhere - you were soaking wet before you even start cycling.
The South Island is reputedly the more scenic one. We saw a lot of temperate rainforest, 2 glaciers, some wonderful coastal stretches and thermal springs. Sometimes we would stand at the beach and look at snow covered mountains. I personally preferred all the apple trees to the rainforest, but that is just my single-minded personal view. By the way: I have never eaten that many apples in my life before than on this trip.
We even saw some interesting wildlife (beside the omnipresent damn sandflies that caused a lot of bit marks around my ankles): Seals! There are huge seal colonies right on the coast line next to a very busy highway (which does not seem to bother the seals at all!). In the end I was happy to move on to the North Island, where the weather was definitely better.