Last month was Aotearoa's warmest September since records began in 1909, according to NIWA.
The nationwide average temperature in September was 11.9C — 1.3C above the 1991-2020 September average.
Temperatures were above average (0.51C to 1.20C above average) or well above average (>1.20C above average) throughout the country, NIWA said in its climate summary released today.
"Exceptionally high temperatures for early spring" were recorded between September 20-21. The maximum temperature of 29.6C was recorded in Wairoa on September 21 — the highest September temperature on record for the North Island, and New Zealand's third-highest September temperature on record. The North Island's previous highest September temperature was 27.7C, recorded in Hastings in 1955, and Waikaremoana in 1975.
The lowest September temperature was -5.9C, which was observed at Mount Cook Airport on September 26.
The "most exceptional heat" was seen on September 20, and was centred on Southland, Otago, and Canterbury. Several locations set new record high daily maximum temperatures for September, including Oban (Stewart Island), which recorded a maximum temperature of 23.5C — 6C higher than its normal daily maximum temperature in February.
Timaru, meanwhile, recorded a maximum temperature of 28.9°C on September 20 — New Zealand's fifth-highest September temperature on record.
Of the six main centres in September 2023, Auckland was named the warmest and Tauranga was the sunniest, while Wellington was the wettest and least sunny. People living further south would have fared better with the higher temperatures, with Dunedin being named the driest, while Christchurch was the coolest.
The sunniest four regions in the country so far this year are wider Nelson with 1859 hours, Mackenzie Basin with 1821 hours, Taranaki with 1809 hours, and Tasman with 1799 hours.
Whakatāne sees record-breaking rainfall
September 2023 was characterised by higher than normal mean sea level pressure over New Zealand, particularly over and to the east and west of the North Island. This produced more westerly and northwesterly airflows than normal, particularly over the South Island.
High pressure systems were more prevalent over the country during the first half of the month, leading to a period of "relatively benign weather" for many parts of the country.
It was followed by more "unsettled" weather in the second half of September, as frontal systems and an El Niño event contributed to "very strong westerly and northwesterly wind events, which are a hallmark of early spring weather patterns in New Zealand", NIWA said.
Rainfall was also above normal (120-149% of normal) or well above normal (>149% of normal) in parts of Southland, Otago, Canterbury, coastal Wairarapa, Gisborne, Bay of Plenty, Waikato, and Auckland.
The majority of the rainfall recorded in the affected areas occurred in the second half of the month, when several heavy rainfall events delivered record or near-record high daily rainfall totals to 19 locations across the North and South Islands.
Tara Hills near Omarama recorded 166mm of rainfall — 441% of its normal amount for September.
Whakatāne, meanwhile, observed its wettest September since records began in 1952, with 287mm of rainfall.
A number of areas received more than double their normal September rainfall, including Gisborne, and inland parts of Otago and Southland. Gore observed 225% of its normal September rainfall, making it the second-wettest September in more than 100 years since records there began.
In contrast, rainfall was below normal (50-79% of normal) in parts of the Hutt Valley, Kāpiti Coast, Manawatū-Whanganui, southern Hawke's Bay, and Banks Peninsula. Rainfall was typically near normal (80-119% of normal) for the remainder of the country.
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