At least four people have died and a state of emergency order continues in Auckland, which on Friday experienced its worst downpour on record.
BBC reports about 350 people needed emergency accommodation, New Zealand's Prime Minister Chris Hipkins said.
He added there had been significant damage across Auckland and the North Island.
The newly-appointed PM also highlighted climate change's role in the extreme weather event.
"It's a 1-in-100-year weather event, and we seem to be getting a lot of them at the moment. I think people can see that there's a message in that... Climate change is real, it's with us," Mr Hipkins said on Monday.
He told national broadcaster TVNZ: "We are going to have to deal with more of these extreme weather events in the future.
"We need to be prepared for that. And we need to do everything we can to combat the challenge of climate change."
Mr Hipkins also acknowledged criticism from locals that communications over the floods had been "too few and far between".
With the unprecedented rainfall Auckland has seen since Friday, even "ordinary" torrential rain in the days ahead could cause more flooding and damage than it would usually, the city's mayor said in a tweet on Monday.
"In parts of the city, the weather looks a bit better - but, don't be fooled, our region is not out of the woods yet," Wayne Brown said.
Many factors contribute to flooding, but a warming atmosphere caused by climate change increases the intensity and frequency of extreme rainfall. The world has already warmed by about 1.1C since the industrial era began, and temperatures will keep rising unless governments around the world make steep cuts to emissions.
The National Institute of Water and Atmospheric Research, the country's climate science body, said Friday was the wettest day on record for a number of locations in Auckland.
Footage and images online showed people trapped in waist-deep floodwater, rescuers carrying out evacuations on kayaks and grocery items floating down the aisles of several flooded supermarkets.