Last week's deluge of rain dumped across the country as part of an 'atmospheric river' was an August record, NIWA says.
NIWA meteorologists say the atmospheric river, a large plume of moisture from the tropics, was the strongest New Zealand has ever seen in August since records began in 1959.
READ MORE: Wellington, Christchurch break all-time winter rain records
It was also the second strongest for winter as a whole, beaten only by an atmospheric river in July 1998.
The rain devastated the top of the South Island, with Nelson experiencing a one-in-120-year rain event and Tākaka receiving a third of its annual rainfall in just three days.
READ MORE: Photos: Long road to recovery begins in flood-ravaged Nelson
The Nelson community has been left to pick up the pieces after the rainfall caused tens of millions of dollars worth of damage.
NIWA meteorologist Ben Noll said last week's atmospheric river was "extremely unusual" for this time of year.
The footage was taken by Nelson Marlborough Rescue Helicopter Trust. (Source: Supplied)
New Zealand usually sees its moistest atmospheric river events in summer, not winter. August is usually the annual minimum for water vapour content that causes the rivers to develop.
READ MORE: Aerial footage shows massive slips in Nelson, Rai Valley
"It was a perfect storm of climate drivers that contributed to this atmospheric river, including La Niña, a negative Indian Ocean Dipole, warmer than average sea water in the Southwest Pacific, and a subsequent weather set up that fostered a long-lived atmospheric river, including a blocking high pressure system to our east," Noll said.
Climate scientist Peter Gibson said that climate change could have a big influence on future atmospheric rivers.
"A warmer atmosphere can hold more water vapour, so it’s likely that New Zealand will see more intense atmospheric rivers as climate change continues. The challenging part will be figuring out precisely which locations will see the biggest increases," Gibson said.
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