A tornado tore through properties in the Waikato yesterday afternoon, ripping up trees, smashing windows and lifting at least one roof.
Intense rain bucketed down in Auckland as slow-moving rain bands crossed the upper North Island.
Forecaster Weatherwatch said the rain would be heavy enough to cause flooding of roads, gutters and basements to Northland, Auckland, Waikato and Coromandel Peninsula.
Aucklanders have been warned not to swim at most beaches for the next 24 hours due to the risk of faecal contamination.
A huge fallen tree also hampered access to a hospital in Auckland.
Meanwhile a tree has crashed onto an ambulance in Whakatane.
Fire and Emergency New Zealand (FENZ) and police attended the scene on Burma Rd, Ohope, after it was reported around 10.10pm.
A police spokeswoman said numerous trees had fallen in the area and emergency services could hear trees cracking in the wind.
No injuries were reported and contractors were being organised to clean up the debris.
Reported tornado causes extensive damage in Waikato
Weatherwatch said a farmer had reported a tornado had ripped off a garage roof and thrown a trampoline across properties, as well as bringing large trees down in Waihou, Eastern Waikato, around 1pm.
Waihou local Jenny McGiven was sitting in her living room when the strong wind and rain hit around 1pm - ripping up trees, breaking windows and destroying outdoor furniture.
"It was just like this wall of wind just hit the house," she told RNZ.
"A couple of panes of glass smashed in leaving glass everywhere. Our barbeque entertainment area is just demolished, we've got outdoor furniture that has ended up paddocks away and numerous trees are broken and down."
Nobody was hurt, she said.
"It's one of those things, nothing you can do about it, you just gotta roll with it."
Rain was coming in the broken windows, she told RNZ. Wind had also lifted the roof of a cow shed nearby.
Road floods, wind brings down trees, powerlines
Earlier high winds battering Auckland brought down a tree across Hospital Rd, completely blocking access to Middlemore Hospital from the north.
Counties Manukau DHB said at 1.45pm the tree had been cleared from one side of Hospital Rd but access was still restricted to emergency services and buses.
Fire crews were called at 11.38am after the tree came down, Fire and Emergency NZ shift manager Craig Dally said.
Three crews are at the scene working at the tree with chainsaws while they wait for council aborists to arrive, Dally said.
There had been no injuries or damage to vehicles but the tree would be blocking the road for several hours, he said.
Fenz had also been called to two reports of power lines down elsewhere in Auckland as stormy weather hits New Zealand.
Power company Vector has placed extra crews on standby in anticipation of power outages.
Elsewhere flooding partly closed Tamaki Drive on Auckland's waterfront earlier this afternoon, the NZ Transport Agency said.
High tide and strong winds had flooded a low-lying section of the road between Ngapipi and Gladstone Rds just before 12pm. All lanes have now reopened.
Rain for 90 per cent of the country on Tuesday
Wind, rain, thunderstorms and even some snow is battering New Zealand owing to a massive Tasman low - though good weather is expected in time for Christmas Day.
The low pressure system could even see small coastal tornadoes forming.
MetService meteorologist Lewis Ferris said the weather system made landfall on Monday night, and would bring rain to 90 per cent of the country as it moved southeast Tuesday.
The hardest hit areas would be Bay of Plenty, the Central Plateau and Taranaki, which all had heavy rain watches in place.
A strong wind watch was in place for Auckland and Great Barrier Island, with potentially gale-force northeasterlies forecast.
And there are also a raft of road closures in the South Island.
Ferris said heavy rain was also expected in Auckland this afternoon, with potential for downpours around rush hour.
"It is something to keep an eye on. There could be heavy squalls over certain areas causing surface flooding, and with the wind add extra element of flying debris, branches falling from trees."
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There was also a moderate risk of thunderstorms this morning and afternoon for Northland, Auckland and northern Waikato, and for this afternoon and evening for the rest of Waikato, Bay of Plenty, western Gisborne, Taranaki, Taumarunui and Taupō.
These thunderstorms could bring localised heavy rain with intensities up to 25mm per hour, strong wind gusts of 90 to 110 km/h and even coastal tornadoes.
For the central North Island there was also the risk of small hail and severe downpours of up to 40mm per hour.
Heavy rain warnings are in place for Mt Taranaki, Nelson and Marlborough through to this evening.
Snow, flood warnings for South Island
MetService had also issued road snowfall warnings for Arthur's Pass, Lewis Pass and Porters Pass for this evening.
Ferris said already elevated parts of Nelson and Marlborough had seen over 100mm of rain overnight Monday.
This evening and tonight there was a moderate risk of thunderstorms about northern Buller and western Nelson, with potential for heavy rain intensities up to 20mm per hour.
The severe weather has already led to rescues in the area.
Two men were winched to safety this morning after finding themselves trapped by the swollen Aorere River in Golden Bay.
Nelson Marlborough Rescue Helicopter chief crewman Paul "Ernie" Bryant told Stuff the pair were caught out when the heavy rain caused the river to rise rapidly.
The pair were uninjured.
Road closures in the South Island include SH63, between Branch River Bridge and the Wairau River Bridge due to flooding.
The NZTA closed the road after continuous rain saw the Wairau Rive overtopping onto the highway.
Around 200m of highway is now under deepening water and can't be driven through safely, agency spokesman Braeden Lobb said.
"Tasman Journeys has closed the road at Korere-Tophouse/SH63 intersection. Crews are also closing the road at Kowhai Point Campsite, on the east side of The Wash bridge."
A soft closure (ie residents will be allowed access to their properties but not through traffic) before the Waihopai River bridge, near Fareham Lane," says Mr Lobb.
This closure is likely to continue into Wednesday due to more rain forecast today in the area.
There is no simple detour route for this highway so people heading for Blenheim or Picton from the West Coast will need to go via Nelson and SH6.
And a temporary speed restriction of 30km/h is also in place on SH1 near the Rangitata River Bridge due to flooding and an uneven surface.
On the West Coast, the planned reopening of SH6 from Harihari to Whataroa will now be weather dependent. The section of the road, damaged by previous flooding, was set to reopen on Friday morning.
Sunny weather forecast for Christmas week
On Wednesday the low would away to the east, directing widespread showers and cooler southwesterly gales to many parts of the country.
Ferris said this would see significant temperature drops, with highs of 13C and 14C forecast in parts of the South Island.
On Thursday and Friday, another front was expected to move eastwards across the country, followed by a narrow ridge on Saturday.
Another front then approaches New Zealand from the Tasman Sea late Saturday.
Although this might all sound a bit unsettled leading into the Christmas break, Ferris said it was unlikely to last.
"At the moment there are pretty good signals there will be quite a bit of fine weather. It looks likely a high-pressure system will cover most of the country. It won't be fine everywhere but no huge rain-bringers at this stage."
Ferris said by Friday they would have a much clearer idea of exactly what weather joy Christmas Day would bring.
While it was too soon to give a full steer on Xmas Day weather, a 10-day automatically forecast generated by MetService's computer weather modelling system was predicting a cloudy day, with a high of 22C in Auckland.
That projection was also predicting a mixture of sun and showers around Gisborne, again with a high of 22C, and sun and showers in Wellington, and a high of 18C.