As we sit here swinging at anchor in Tonga, at the end of October, boats are in transit, already transited or waiting for weather to sail to New Zealand.
One reason New Zealand is the default option is because it is commonly considered to be outside of the cyclone belt...but is it?
Yes. By definition, a cyclone* ceases to be a cyclone the moment that it leaves the tropics. This exit is called an extratropical transition and the former cyclone is now called “a storm of tropical origin”. In addition to the fact that a cyclone has exited the tropical arena and reaches colder waters, it often (but not always) loses other physical characteristics such as the eye that are necessary to be considered a named cyclone.
No. Even though this storm of tropical origin is no longer accurately labeled a cyclone, mariners and news outlets usually continue to use the official cyclone name as a former cyclone approaches New Zealand because, semantics aside, the strength of the wind in the storm of tropical origin can be as extreme as any tropical based cyclone. Over a 27 year period (1970-1997), Northland averaged one storm of tropical origin, with cyclone strength winds, per year. In April 1968, Tropical Cyclone Gisele re-intensified near Wellington producing winds gusting to 145 knots and blowing the roofs off of houses as far south as Christchurch on South Island. As recently as March 2014, Cyclone Lusi brought cyclone strength winds to North Island.
It Depends. These former cyclones, as they reach New Zealand, are usually rapidly weakening and for that reason don’t often reach very far down the North Island. While an average of one extratropical cyclone per year hit the area from Auckland north, the area south of there, between Auckland and the top of South Island averaged half as many. Still, extratropical storms of considerable strength, as described above, have reached all of the way down South Island. Strangely enough, although El Nino is generally associated with increased cyclonic activity, the strongest extratropical storms affect New Zealand when the ENSO cycle is neutral, not El Nino.
In Sum. While the risk is still relatively low, New Zealand is not accurately described as being unaffected by cyclone strength storms of tropical origin. New Zealand has been directly hit by substantially more extratropical storms of cyclone strength than areas of French Polynesia (i.e., the Marquesas and Gambiers). It is more accurate to say that New Zealand is a low risk border region – albeit one with well protected harbors, stout marinas and facilities for repairing any damages.
*In this region a cyclone, among other characteristics, starts as a 10 min sustained wind speed of over 34 knots. 10 min sustained wind speeds of over 64 knots (a hurricane in other regions) are a Category 3 cyclone.