Auckland's been soaked by a month's worth of rain in the past 24 hours, earlier leaving nearly 20,000 homes without power, and causing roads closures and slips.
A thick band of rain has been pounding the city, region and North Island.
Metservice meteorologist Lisa Murray said the areas had been experiencing some thunder and lightning, as well as bouts of extremely heavy downpours.
"This rain band is affecting Auckland, extending all the way up to Northland, and down almost as far as Hamilton.
"It's moving to the west, so it will ease off this evening.
"That's the characteristic of a system like this, it comes in bands.
"So there are more bands of rain that are due to come in later, as well."
Power was restored to most of South Auckland on Thursday night after about 19,000 properties were affected by a fault with Transpower equipment from 3.40pm to just after 5pm.
About 200 homes around the Auckland region remained without power because of issues caused by weather, a Vector Energy spokeswoman said.
Most were caused by trees falling on to powerlines or power poles falling over in loose ground or where there were slips.
Vector had hoped to have power restored to most homes tonight but said technicians had trouble reaching affected areas in Clevedon, Tapora and Takanini and work there was likely to go on into the night.
Vector were working with the council to access the areas blocked by slips.
Other outages were in Omaha, Wellsford, Mairangi Bay and Rosebank.
A severe thunderstorm watch is in place for the Auckland region, and squally gusts of up to 110km/h are possible.
Whangaparaoa has recorded 167.8mm over the last 24 hours, while Auckland Airport has recorded 82.4mm.
Auckland usually gets 91mm of rain for all of April.
"This is a significant event, by any standards," Murray said.
Power cuts also affected about 60 traffic lights.
The New Zealand Transport Agency said at the time traffic lights were out between Hill Rd and Highbrook Drive.
Police officers had manned a small number of intersections but asked drivers to take care at other intersections.
There did not appear to have been any crashes caused by the outage.
Both the Manukau train station and the Manukau Mall were also without power
Auckland Council head of emergency management operations Aaron Davis said further slip and flooding damage was possible across the saturated city.
"If you don't need to travel don't go out," he said.
The overnight deluge has left many parts of the region saturated with widespread road closures, houses threatening to topple into the sea and massive landslides.
Motorists were advised to avoid travelling through flooded roads.
Other measures to minimise damage were to unblock drains and help neighbours who may be struggling with flooding.
This rain band is moving into the Bay of Plenty this evening and is expected to be severe overnight, easing off around dawn.
The researchers aim to create early detection tests, improving survival rates.