But, but, if the North Island and the South Island are the same country, why is not the West Island??

How far apart do you think the North and South Islands are? As a child I was fascinated to discover that a sizeable chunk of Te Ika a Maui lies South of the northernmost tip of Te Wai Pounamu. People swim between the North and South Islands aon a reasonably regular basis, the last being a 14 year old. I am not sure how many have swum from here to the West Island.