MetService is warning people on the West Coast to be wary of powerful waves on New Year's Day, while western areas of the central North Island could get a burst of heavy rain. A New Year's Day swell pulse was generated by gales south of Australia and arrived during the night in the south of the South Island, MetService said. "As the winds generating the waves are far away, the waves with the longest wavelength arrive first. The energy in a wave is proportional to its wavelength, so these waves are also particularly powerful." After lunch thunderstorms are possible for much of the North Island, except Kāpiti, Wellington, southern Wairarapa, and coastal Gisborne and Hawke's Bay.

Source: Stuff December 31, 2017 21:51 UTC