Kumeū in West Auckland has experienced its second wettest day since records began as a storm flooded homes and cut off roads.
Niwa says the area copped 201mm in just 14 hours - and a staggering 149 per cent of the August monthly normal rainfall fell in a single day.
Firefighters have resorted to using jet skis to reach trapped motorists as the township on the Twin Coast Highway is inundated.
Kumeū, Huapai, Rānui, Piha and Henderson Valley appear the worst hit as Civil Defence assess the damage.
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• Wild weather: 50 homes evacuated in west Auckland, houses flooded, cars underwater, roads cut off
A witness said this morning it was "absolute chaos".
"There's one car parked with water up to its windows. There's a stream running off the buildings and footpaths, cars are floating around," he said.
"I've never seen anything like this."
Auckland Mayor Phil Goff said he was hearing 50-60 homes could be evacuated.
People had largely been self-evacuating and staying with family and friends but there was a shelter in the council building in Henderson.
West Auckland locals who have been evacuated from their homes and need somewhere to go can go to the old council buildings at 6 Henderson Valley Road, where an evacuation hub has been set up.
"I'm imagining that some people will need accommodation...we may need to make provision for people," Goff said.
He told RNZ it never rains it pours.
"We've been dealing with drought and now we've got this.
"It's pretty dramatic after three years when we have been suffering drier than usual conditions and sometimes drought, to get this sort of deluge," said Goff.
The news was "traumatising" for people who had already been dealing with lockdown.
Goff told the Herald that the massive downpour was the biggest since the Tasman Tempest in March 2017, which delivered isolated instance of once-in-a-century rainfall. In one day, Auckland recorded its wettest March day on record 10mm - since 1959.
Goff said last night's downpour was a huge 90mm an hour and parts of Kumeu/Huapai, Piha and Bethells Beach are still getting 40mm an hour.
He said last night's downpour was a huge 90mm an hour and parts of the city are still getting 40mm an hour.
The council has set up an emergency shelter at 6 Henderson Valley Rd, in Henderson, and would provide accommodation in motels to people who have been forced from their homes, the mayor said.
Goff said he planned to travel from his office in the central city to West Auckland by mid-morning, conditions allowing.
A bonus of the storm, he said, was the level of Auckland's water lakes had risen from 67 per cent to 72 per cent full.
The 19-year-old disappeared without her wallet or mobile phone.