The North Island is one of the two main islands of New Zealand, the other being the South Island. (A grammatical note: New Zealanders refer to them with a definite article as "the North Island" and "the South Island", like "the North Sea" and "the Western World", but unlike "Rangitoto Island" or "West Point"). Several important cities are in the North Island, notably Auckland and Wellington, the capital of New Zealand at the southern extremity of the island.
The story goes that he and his brothers were fishing from their canoe (the South Island) when he caught a great fish and pulled it from the sea. While he was not looking his brothers fought over the fish and chopped it up.
This great fish became the North Island and thus the Maori name for the North Island is Te Ika O Maui (The Fish of Maui). The mountains and valleys are said to have been formed as a result of them hacking at the fish.