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The Nelson area has been hit with the second highest rainfall ever recorded in one hour.
Flash flooding has occurred across the Tasman region with roads closed, motorists trapped and homes flooded.
Over the course of one hour this evening, 104mm of rain landed in the Upper Roding catchment and 100mm in Stoke. The heaviest rain received within one hour on record was 109mm in Auckland, Civil Defence said.
There were abandoned cars on roads throughout the affected areas and some motorists had to be rescued or towed to safety.
Police said they were dealing with a high number of flooding reports in the Richmond and Stoke areas and emergency services were responding to several incidents, including stranded vehicles and property flooding.
The rain had eased but residents were warned more rain, of about the same intensity, was expected overnight.
An emergency operations centre (EOC) has been established at Tasman District Council in conjunction with police, fire and both local councils.
Although the centre was not aware of any evacuations so far, it was setting up a welfare centre at the Richmond New Life Centre for any residents who needed to get out their homes or who who would be evacuated.
Some houses had been flooded already.
Residents were asked to stay indoors where possible and not to travel on any roads.
One road had so far - Kina Beach Road where the bridge had been lifted - had been closed by authorities.
Other roads had been rendered impassable because of landslides or flooding, including the Takaka Hill highway.
WeatherWatch.co.nz said a band of rain that had stalled over the region was responsible for the deluge.
Similar showers and bands of rain were moving in to Waikato, Auckland and Northland, the forecaster said.
After also wreaking havoc in the Western Bay of Plenty, the forecast severe weather has hit Hamilton.
A MetService alert says there is a potential for severe thunderstorms with large hail and damaging winds.
Newstalk ZB reporter Sophia Duckor-Jones says the water level is rising in her street in Hamilton East.
"We've got a car parked out on the road that we're going to have a bit of trouble moving.
"In parts of the street, the water levels are definitely over my ankles. It's raining really, really heavily, lots of thunder.
"If I was to describe it to you perfectly, it looks like the street in front of me is just being swallowed up by the rain."
"Explosive" torrential showers and severe thunderstorms are possible over much of the North Island today, WeatherWatch says.
The forecaster was tracking areas of rain which it said were similar to those which caused severe flooding in parts of Bay of Plenty yesterday.
"Residents are advised to stay indoors during these downpours - similar set-ups produced the Hobsonville and Albany tornadoes" it said.
"In saying that it's important to put this into perspective: while the risk of a tornado may be high, the chance of one hitting your home is not."
The primary focus should be on flooding caused by torrential showers.
In Raglan, a small tornado brought down a tree, broke windows and hurled a barbecue onto a roof about midday.
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Raglan Constable Jeremy Holmes estimated the tornado had caused damage over a two-block radius.
"Quiet a few big branches have been broken off, there was a chimney down on a property, which also had a couple of popped windows."
Fire Service spokesman Scott Osmond said emergency services received three calls about the tornado from Taipari and Tahuna Ave around midday.
No one was thought to have been injured.
WeatherWatch has issued storm alerts for Auckland, Northland, Waikato, Taranaki, King Country, the Central Plateau, Coromandel and the western Bay of Plenty.
New Zealand Transport Agency Canterbury-West Coast state highways operations manager Pete Connors said the weekend's heavy rain had resulted in numerous minor slips on State Highways 1, 7 and 73.
There had also been some localised flooding on the outskirts of Christchurch.
Dozens of people were evacuated from Waihi Beach yesterday as flooding of up to 70cm swamped the seaside town and washed out a bridge.
A section of Waihi Beach Rd was also washed out and police expected it to remain shut for the next few days.
MetService meteorologist John Law said the wet weather should continue over much of New Zealand through the early part of next week.
Bay of Plenty west of Whakatane continued to be in the firing line today, with up to 200mm of rain and thunderstorms possible near the coast.
"Moderate to heavy rainfall is likely throughout Bay of Plenty, not just in the western part of the region, again on Sunday and Monday."
Heavy rainfall, sometimes thundery, was also expected in parts of Taranaki and the Tasman and Marlborough Districts through to Monday.
Moderate to heavy rain was also expected in the Canterbury high country and foothills and the ranges of eastern Otago through to later today.
- additional reporting NewstalkZB