NIWA's chief climate scientist says it is extremely likely that last month's powerful storms were the result of climate change.
Dr Sam Dean, speaking this morning to TVNZ 1's Breakfast programme, said it is undeniable that greenhouse gas emissions have raised Earth's average temperature over the past 100 years.
High sea surface temperatures in the Tasman Sea have contributed to the power of the storms, he said.
"This is just so unusual ... we're really just in uncharted territory here - this is something we haven't seen," Dr Dean said.
January was the hottest month ever seen since records began, Dr Dean said - more than 100 years - and the chances of storms like last month's occurring naturally are very low.
Dr Dean said natural variation in the seasons was common, with spikes and dips in extreme temperatures, but the data clearly supports the fact that higher temperatures are affecting the weather.
"It's not really a question of belief, it simply is a question of fact, and that's how it is," he said.