Perhaps this will help...btw, Mike C, good response.

How often is New Zealand hit by tropical cyclones?
by Mark Sinclair

"Recent NIWA research shows that northern New Zealand is hit by an average of a little over one storm of tropical origin each year. The severity of these storms depends on their location and on the phase of the El Niño/La Niña cycle.

Every year between December and April, storms from the tropics move south toward New Zealand. As these tropical cyclones advance poleward, they eventually lose characteristic hurricane features such as the eye and the surrounding symmetric cloud and precipitation region and progressively acquire the asymmetric cloud and thermal features more typical of a frontal mid-latitude storm. Scientists use the term extratropical transition (ET) to refer to this poleward movement and accompanying structure changes.

As these storms move into New Zealand waters, they often maintain sufficient vigour to produce damaging winds, high seas and heavy rain. Occasionally, tropical cyclone remnants re-intensify in the extratropics to become potent mid-latitude storms capable of inflicting loss of life and severe property damage.

Several memorable ET events have occurred in the New Zealand region. In April 1968, Tropical Cyclone Gisele re-intensified as it moved south over New Zealand, producing winds gusting to 75 m/s (270 kph) in Wellington, and sinking the interisland ferry Wahine with the loss of 51 lives. Cyclone Bola dumped over 900 mm of rain and produced hurricane-force winds in regions of northern New Zealand in March 1988. More recently, Cyclones Fergus and Drena brought torrential rain and storm-force winds to the North Island in December 1996, triggering an exodus of summer tourists from coastal resorts.

Cyclones near New Zealand

Frequency by month of tropical cyclones progressing south of 35°S from 1970–1997 data. The total number of storms was 81.

In the Tasman Sea, west of New Zealand, average storm motion is only about 5 m/s due south. On the other hand, east of New Zealand, tropical cyclones move away to the south-east at speeds exceeding 8 m/s. Average tropical cyclone intensity is greatest near latitude 20–25°S, indicating that most storms moving into New Zealand waters are weakening. Storms that undergo ET in the Tasman Sea are more vigorous than those east of New Zealand.

The largest numbers of tropical cyclones make it into middle latitudes (south of 35°S) in February and March. If we compare these events with overall cyclone numbers (which also peak in February), we see that March is the month during which the greatest fraction of storms will take a track into middle latitudes. Put another way, this means that, given a tropical cyclone, it is most likely to move south of 35°S in March. This corresponds to the time of warmest sea temperature, a factor that scientists have shown to be essential for tropical storm survival.

An ENSO connection

The location and intensity of ET is also affected by the phase of the El Niño–Southern Oscillation (ENSO) cycle. During El Niño years, tropical cyclones enter middle latitudes anywhere between the Australian coast and about 150°W and typically weaken quickly as they move away to the south-east.

During La Niña years, ET is largely confined to longitudes west of the dateline (longitude 180°). These storms move more slowly toward the south and tend to retain their vigour for longer. Interestingly, the strongest storms appear to affect New Zealand when the ENSO cycle is between El Niño and La Niña."

LI Phil