Severe Weather in New Zealand

           Tornadoes form in supercell thunderstorms, which are larger and more severe than regular thunderstorms.  Air starts to rotate because of the winds at different altitudes blowing at different speeds creating a wind shear. This horizontal rotating air is then lifted off the ground by the updraft of the thunderstorm. Once the horizontal rotating air is lifted nearly vertically, it is called a mesocyclone. The mesocyclone is then fully developed in the updraft. This can lead to a funnel cloud, which is right before a tornado forms. A tornado forms once the funnel cloud comes in contact with the surface.

In the United States, tornadoes usually travel from west to east or more specifically, southwest to northeast. This is the dominant direction for tornadoes because wind direction in general for the U.S is from west to east and this wind is called the Westerlies. Tornadoes are very scarce in New Zealand, but they do happen. The average number of tornadoes observed in the US per year is a whopping 1,000 (Source). The average number of tornadoes observed in New Zealand per year is 8, which is extremely low and good (Source). 

http://www.ncdc.noaa.gov/climate-information/extreme-events/us-tornado-climatology 

In the map above, you can see that New Zealand is in the hot spots of tornado activity. Although on the graph of tornado occurrences over the years showing an increase in annual tornado count, it may not be true. The reason being is that a hundred years ago, not too many people knew what tornadoes were.  Technology has now helped more people understand tornadoes then being able to report more of them.

The three main requirements for a hurricane is a warm ocean temperature more than 80 degrees F, deep warm ocean layer about 200 meters in depth, and coriolis to initiate the spinning of the air. They need to be between 5 degrees N and S. New Zealand does not satisfy these requirements because the water is too cold being fairly far away from the equator and they are too far south for the coriolis to be strong enough. The main hurricane formation regions are the Atlantic, East Pacific, Indian Ocean near Australia, off the coast of China and Indonesia. They are termed hurricanes in the Atlantic and Pacific; tropical cyclones in Australia; and typhoons in Indonesia. The general direction hurricanes travel is up the warm Gulf Stream because of the west to east flow of the Westerly winds and jet streams.

http://www.learnnc.org/lp/editions/nchist-recent/6248

Tropical cyclones (hurricanes) do happen in New Zealand, but are very scarce! On average, ~1.75 hurricanes hit the U.S every year (Source). I couldn't find the average number of hurricanes to hit New Zealand per year, but I did find that New Zealand is hit by an average of a little over one storm of tropical origin each year. This is not the same as a hurricane and is a weaker storm system (Source)