More than 100 lightning strikes and rainfall of up to 100mm in some areas hit Northland, causing flash flooding, while more heavy rain and thunderstorms are expected today.
MetService has warned that severe thunderstorms and torrential rain of between 30mm and 50mm, especially in parts of the Far North, could cause further flash flooding and slips.
Weather forecaster Daniel Corbett said Northland had more than 100 lightning strikes within two hours yesterday morning, with about half over land.
"There's a fair bit of frequent lightning in them so it's certainly an active weather system, and there's a risk of lightning going up towards the end of the day," he said yesterday.
The worst flooding in the Mid North as of yesterday afternoon was on Te Ahu Ahu Rd, near Waimate North, where the road was covered at two locations by rapidly rising tributaries of the Waitangi River. The floodwaters were fast flowing but still passable by car.
Police and contractors were called out to put up warning signs and advise motorists.
One local said the valley filled up every time it rained but he had never seen the river rise as quickly as it did yesterday.
Elsewhere, State Highway 10 near Kerikeri and Wiroa Rd and Valencia Lane near Bay of Islands Airport were partly flooded but remained passable.
Mr Corbett said areas to watch were along the east coast between Whangarei and the Bay of Islands.
An active trough moving southwards across Northland accompanied by a band of heavy rain was expected to ease off from today.
As of 3pm yesterday, the main road through Kaeo - usually the first highway to disappear under water - was still "bone dry", Kaeo Farm and Fuel owner John Owens said.
Locals would keep an eye on the river as high tide approached at 6.45pm last night but were not overly concerned, he said.
According to Northland Regional Council data, Ohaeawai was by far the wettest place in the Mid North, with a 95mm drenching in the six hours from 6.45am to 12.45pm yesterday.
The heaviest downpour per hour, 29.5mm, was between 9.45 and 10.45am.
Over roughly the same six-hour period the weather station at McDonald Rd, near Oromahoe, copped 44.5mm and Otiria 23.5mm.
In the 12 hours to 2pm yesterday, central Whangarei had received 7.8mm of rain, Wilsons' Dam 8.5mm and Paparoa in Kaipara 13mm.
Northpower, the lines company for Whangarei and Dargaville, and Top Energy in the Far North, did not experience any power outages late yesterday as a result of the lightning strikes but were monitoring the situation last night.