An 'unprecedented' El Niño could develop later this year, the country's climate forecasting service says.
Earth Sciences New Zealand (ESNZ) is holding off formally declaring an El Niño event but says that is likely to change within the next few weeks.
Chief weather scientist Chris Brandolino said the conditions developing in the tropical Pacific Ocean had all the makings of one of the strongest El Niños on record.
"It is distinctly possible that this El Niño may comfortably exceed the previously high-end El Niño event."
An El Niño generally means wetter conditions in the south-west of the South Island, and drier, windier conditions in most other parts of the country, especially the east and north-east.
That increased the odds of adverse conditions like drought and wildfire but did not guarantee them, Brandolino said.
"While we're pretty confident we will see something quite extraordinary, potentially unprecedented in terms of El Niño strength, we're not quite so certain that those impacts will follow suits here in New Zealand."
"They may, but that confidence level isn't quite so high because there are other things that influence our weather."
However, he sounded a particular warning for Canterbury, which was already in meteorological drought.
"They're already on the back foot."
Yesterday, Australia's Bureau of Meteorology became the latest Pacific agency to officially declare an El Niño had formed.
The Japan Meteorological Agency and the US National Ocean and Atmospheric Administration both declared an El Niño event last week.
Agencies draw on the same data and models but use slightly different thresholds and criteria to determine if an El Niño is in effect, including the climate or weather conditions that are being observed in the agency's own country.
Brandolino said ESNZ was keeping a close eye on how conditions matched up against the criteria it used, but was expecting to make a formal declaration soon.
The swing from La Niña at the start of the year to a developing El Niño less than six months later was "quite remarkable", he said.
El Niño forms when there are higher-than-usual temperatures in the eastern and central Pacific, causing trade winds to die down. That means clouds form and rain falls on that side of the ocean, rather than closer to Australia and New Zealand.
Ocean temperatures in the central Pacific had gone from about 1°C below average to 0.7°C above average since January, Brandolino said.
"The ocean has done this remarkable about-face and transition."
It was common for the Pacific to spend month or years in a neutral phase, but that had not happened, he said.
"It just cruised by neutral."
The five strongest El Niño events in modern history had been (in chronological order): 1972/73, 1982/83, 1992/93, 1997/98 and 2015/16.
"This El Niño is forecast to rival some of the more intense El Niño events," Brandolino said.
"A very strong El Niño or so-called Super El Niño is when ocean temperatures, the anomalies, reach or exceed 2°C ... and this event is forecast to comfortably exceed that."
Some models showed the ocean temperature anomaly surpassing 3°C above average.
Brandolino warned against a false sense of security in regions affected by heavy rain earlier in the year.
"Areas that have received a fair amount of rain over the past six months because of La Niña, those areas are likely to become dry."
Higher ground moisture in the upper and eastern North Island would likely buy those places some time, though.
"But the eastern South Island, they don't have that luxury - they're already in very dry conditions."
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