The infamous "Giselle/Wahine" month. Various papers and analyses followed over the years. An extract from Erick Brenstrum's "The New Zealand Weather Book" gives an excellent description of the weather involved:
https://www.iwonderweather.co.nz/story/Wahine/6356/topic/forecastingApril 1968: The weather for April was dominated by a tropical storm which originated near New Caledonia. It crossed the North Island on the night of 9th-10th and moved down the Canterbury coast on the 11th. Violent gales caused damage over the greater part of the North Island and the northern half of the South Island. The greatest destruction from wind was in and around Wellington, where southerly winds averaged as much as 80 mph on the morning of the 10th, with gusts to 125 miles per hour; while on exposed hilltops gusts were recorded to over 150 mph. Thousands of trees were uprooted, and roofs were blown off houses. During the storm the inter-island ferry Wahine ran aground at the mouth of Wellington Harbour. Flooding occurred in many areas, notably around Christchurch and Banks Peninsula on the 12th and in Southland and West Otago on the 15th.
Rainfall: Rainfall was above average or close to average over almost the whole country. It was more than double the average on the east coast of the South Island and up to about 50 miles inland from Balclutha to Blenheim; north of a line from Hamilton to Tauranga; and also over the greater part of the area south of a line from Stratford to Waipukurau. Around Lyttelton and Banks Peninsula some stations recorded 5-8 times the average value, the highest totals there being
27.69” at Hickory Bay in the east, and 20.46” at Purau, on Lyttelton Harbour. In these areas at least 2/3 of the monthly total was recorded on the 10th and 11th, amounting to 16.21” at Hickory Bay and 14.33” at Purau; while many other stations had more than 8” for these 2 days. The highest totals for the month were recorded at two stations in central Northland, Tau’s Falls with 31.05” and Puketi with 30.71”. At Puketi the total for the first 3 days of the month was 11.86”, for the first 9 days 28.52”
and for the 14-day period 27 March–9 April 33.8”. Flooding was reported in Northland on the 3rd and again on the 9th-10th. Heavy rain caused flooding in Buller and North Westland on the 9th.
The Southern Lakes district received less than half the average rainfall, and it was also appreciably drier than usual in Fiordland and over the greater part of the Alps.
Temperatures: Temperatures were 1F-2F below average over the South Island and 1F-2F above average over the southern half of the North Island. The first 9 days were exceptionally warm for April.
Sunshine: Sunshine was below average over most of the country. Greatest deficiencies of up to 50 hours were recorded in Canterbury and Nelson, also in parts of the central North Island. Nelson, Hanmer, Christchurch and Lake Tekapo all had their lowest April sunshine since 1938.
[Some totals: Kaitaia 149, Kerikeri 140, Whangarei 127, Leigh 134, Auckland 122, Tauranga 137, Whakatane 167, Taupo 135, Opotiki 152, Hamilton 134, New Plymouth 132, Masterton 114, Dannevirke 118, Gisborne 144, Napier 158, Wairoa 138, Paraparaumu 140, Ohakea 148, Palmerston North 123, Levin 148, Wellington 118, Stratford 122, Ohakune 82, Wanganui 139, Westport 132, Hokitika 130, Greymouth 105, Haast 149, Riwaka 140, Nelson 136, Blenheim 173, Grassmere 133, Kaikoura 101, Mt Cook 86, Mt John 124, Methven 102, Christchurch 95, Tekapo 117, Timaru 87, Waimate 99, Omarama 132, Dunedin Airport 106, Dunedin 97, Oamaru 98, Queenstown 91, Alexandra 129, Invercargill 117, Campbell Is 57 {22 above average}].
Sources: NZ Met. Misc. pub. 107 (Climatological table, NZ Gazette) and NZ Met. Misc. pub. 109 (Meteorological Observations) - additional comments by the poster.
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September 06, 2018, 07:51:54 AM