A group of local and international scientists say climate change played a role in the devastating rainfall from Cyclone Gabrielle that claimed lives and wrought massive destruction.
Their rapid analysis - the first time something like this has been done - found human-caused warming was driving increased rainfall, and made extreme rainfall events more likely.
Niwa principal climate scientist Dr Sam Dean said a staggering amount of rain had fallen.
"This was a gigantic, gargantuan event and I have no doubt whatsoever in my mind, across my experience of my life as a climate scientist, that climate change has influenced that event."
Dean is part of a group of local and international researchers who have found the warmer planet has made very heavy rain like the cyclone 30 percent more intense, and about four times more common in the region.
He has some discomfort about such concrete figures - their study was done at speed, and the amount of data available and complications with climate models means their was variability in their findings.
But the results were bolstered by rigourous analysis and were backed by previous studies.
The scientists could find no plausible explanation other than human-caused warming for the observed increase in heavy rainfall.
University of Waikato environmental science senior lecturer Luke Harrington said: "We're confident that the amount of rainfall associated with Cyclone Gabrielle was more intense because of anthropogenic climate change.
"But we cannot yet provide a precise answer as to precisely how much more often we will witness similarly intense rainfall events."
Victoria University Climate Change Research Institute professor Dave Frame said after extreme weather events there was often speculation about the role of climate change.
The faster you can bring scientific evidence to bear the better, he said.
"To use Churchill's old line, you know it's a question of the truth getting its pants on faster, you can actually get out there and and lock off some of the silliness.
"Even if later we do more studies and revise things and develop a more mature understanding, this sort of thing can can help with that first response."
Meanwhile, Red Cross Red Crescent Climate Centre advisor Julie Arrighi said human decisions about where to do farming and forestry also made the cyclone damage worse.
"Land use changes that reduce soil stability or combined with deforestation can increase runoff and contribute to impacts."
Imperial Collage London researcher Friederike Otto said it showed the need to mitigate, and reduce emissions.
"It means that every additional bit of warming will make these kind of events worse.
"And therefore, I think showing that climate change is not something that happens sometime in the future or to someone else, but actually affects people ... everywhere around the world today.
"And that therefore it's really not something that we can debate about for another 30 years before starting to act."
After huge events like this, there was a window to make changes to reduce the risk in the future, Otto said.
While this study had not been peer reviewed, the methods and protocols used to generate the findings had been, she said.