A new record for heavy rainfall has been set by this week's torrential downpours.
Heavy rain hit the upper part of the country overnight on Sunday and then landed in Auckland, reaching the heaviest on Monday morning.
Numerous homes and businesses were flooded, with more than 300 calls for help to Fire and Emergency, and widespread road closures across the upper North Island.
Niwa said the 103mm of rain that fell between 4am and 5am at Maungatapere near Whangārei on Monday was a new hourly record for a low elevation station.
The previous record was set 56 years ago when 100.6mm fell at Whenuapai in Auckland's northwest. A low elevation station is one positioned up to 500m above sea level.
That highest-ever record for rainfall at any elevation in New Zealand is held by the Cropp at Waterfall high elevation station near Hokitika, which recorded 134mm of rain in an hour in 2004.
Niwa meteorologist Chris Brandolino said Auckland and the surrounding areas usually get up to 90mm of rain through the entire month of March.
Hourly rainfall rankings at Niwa are calculated by the hour, always measuring from "the top-of-hour to top-of-hour".
The maximum 60-minute total recorded at Maungatapere on Monday morning was actually higher than the new official record, as the station reached 123.2mm between 3.30am and 4.30 am.
"Because off-hour totals are not routinely calculated or cited in records, we can't definitively confirm whether this would represent an off-hour record, although it did exceed the previously known off-hour record of 109.4mm at Leigh, Auckland in May 2001," Brandolino said.
But both measurements are astounding, he said.
"For 103mm to fall in one hour, that's exceptional - you're getting more than what would fall in a month in one hour.
"You can see why we had the flooding Monday morning across much of parts of Northland, and much of the Auckland region."
Brandolino explained that on Monday when the record occurred a low pressure system in the Tasman Sea was siphoning moisture from the tropics into the northern North Island.
"Around 3000 lightning strikes were observed in Northland and immediate offshore waters on Monday morning; this thunderstorm activity increased the rate of rainfall.
"Ongoing marine heatwave conditions around New Zealand may have also contributed to the heavy rainfall."
Later in the week deluges hammered Hawke's Bay, where roads, bridges, farms and houses were damaged, with repairs expected to take at least a year.
Brandolino said the weather system that caused so much damage around Auckland, Gisborne and the Hawke's Bay was especially slow moving because its progress had been stalled by an area of high pressure to the southeast.
"A warming planet means that we expect to see more extreme weather events like this. In the future, it's likely such events will become even more common and more extreme."