I think that the best time for tourists to come visit New Zealand is around summer. I chose that season because it is around Christmas and new year, and they are special time for most New Zealanders. They can go for a ride on a fairy boats, travelling to Milford sounds and either going to the beach or the pools.
They can also go up to Capereinga which is the very top of the north island in NZ. They can visit Auckland because it is the biggest city in, it has the tallest tower in, and the longest bridge and tunnel. They can visit Wellington because it is the capital city in NZ and has the parliament building.