You can spot whales in the North Island, but it's Kaikoura in the South that is the main centre for whale watching in New Zealand.

Kaikoura, on the east coast of the South Island, is one of the only places in the world where you can easily see sperm whales.

Sperm whales, the largest of the toothed whales, grow to over 15 metres in length. They dive deep into the ocean to feed. The resident population of sperm whales at Kaikoura can be seen all year round. Orca (killer whales) may be seen from December to March, and humpback whales in June and July. Several dolphin species are seen almost daily in the area.
 
Whales thrive close to Kaikoura because of its unusual submarine landscape. The continental shelf drops quickly into a number of extremely deep underwater canyons. In addition, a warm current from the north meets a colder one from the south. This causes nutrients from deep within the ocean to be carried upward, a phenomenon that helps to support all types of marine life from plankton and krill to dolphins and whales.