January was Auckland's wettest month in history, according to NIWA.
The region has been hit hard by extreme weather and flooding, forcing many to evacuate their homes.
During the storm, NIWA said they recorded 539mm of rain at Albert Park, a significantly higher number than the previous monthly record of 420mm in February 1869.
Anthony Fowler from the School of Environment at the University of Auckland said the amount of rain Auckland has seen in the past month is "astounding".
"Total January rainfall was more than twice the previous record for the month, contributing to an exceptionally wet summer and the wettest three months on record," he said.
"It was already a very wet January before the storm on the 27th, but that single event doubled the total over a single day. Rainfall intensities in the early evening, at the height of the storm, were quite astounding."
NIWA said the rainfall was at its worst on Friday, with Albert Park being pummelled by 280mm of rain in 24 hours and 211mm in six hours.
They called it a "one-in-200-year" weather event.
Across January, the region experienced 45% of its yearly rainfall — around eight-and-a-half times more than the usual 1190mm it receives in the first month of the year.
NIWA meteorologist Ben Noll said several elements coming together were responsible for the significant deluge.
"A slew of environmental factors contributed to this extreme event — a formidable La Niña and marine heatwave led to more moisture being available, which was harnessed by an atmospheric river. High pressure to the south then blocked it, keeping it in place.
"The storm was also supported by a unique phenomenon called a low-level jet, as well as converging winds that extended lengthwise across the most populated part of the country."
The weather was also made more intense by the effects of climate change.
NIWA climate scientist Sam Dean said the earth has warmed by about 1.1C already and believes climate change contributed 10% to 20% during the storm.
"This feeds into a proportionately larger effect on the extent and depth of flooding, causing more damages and costs," Dean said.
"It's imperative to consider whether climate change has increased the likelihood of the storm's precursor conditions occurring again — will these events become more likely as tropical weather expands southwards? If so, New Zealand must be able to adapt."
SHARE ME