Despite increased variability, hot, dry conditions could present fire risks along the east coast of the South Island, from the top to Central Otago.
And the arrival of a strong El Nino will bring "really windy conditions" that could push fire through green pastures.
Niwa meteorologist Ben Noll said New Zealand’s hottest temperature happened during an El Nino summer — when temperatures climbed to 42.4°C in Rangiora in February 1973.
This El Nino event, confirmed last week, would be one of the biggest in the last couple of decades.
The weather pattern would "elevate and enhance" the variability of spring, which could produce wild swings in temperatures "and it’s going to bring some really windy conditions".
Niwa forecasting principal scientist Chris Brandolino said it was more likely than not that temperatures would climb to 40°C in parts of the country over summer.
"When you superimpose these warm phases of natural variability, El Nino, on to a warming climate it’s hard not to think, ‘How do you not get warmer?"’
University of Otago geography senior lecturer Dr Daniel Kingston said the El Nino influence on top of the region’s climate-change-affected summers was adding "another layer of extremeness".
As global temperatures increased over the coming decades, the hot days were going to get hotter as well.
"What we might think of as an extreme hot day at the moment will occur more regularly into the future.
"If we used to expect El Ninos to make our summers that bit drier or that bit hotter, under climate change we are starting from a higher baseline."
El Nino would produce more westerlies in particular, Dr Kingston said.
While the West Coast could receive a lot of rain, it was the eastern side of the South Island that tended to produce drier conditions.
Fire and Emergency NZ national wildfire specialist Tim Mitchell said strong winds could push fire through green paddocks as it burned through thatch.
Through a variable spring, he expected "a see-sawing of fire danger risk".
Heading into Christmas though as the land dried out there would be increasing risk in the east coast of both islands, down into Central Otago, he said.
"Normal is quite different to what we’ve experienced for the past couple of seasons. This year is going to be different."
