Cyclone Hale has made landfall on the North Island's East Coast with gale-force winds, 6m swells on the coast and heavy rain.
It swept over Northland, Coromandel Peninsula and Great Barrier Island on Tuesday morning, and is forecast to reach Marlborough and the North Canterbury coastline from Wednesday morning.
MetService said Northland and North Auckland have had nearly a month's worth of rain in the past 24 hours.
The rain is predicted to peak in most areas on Tuesday afternoon, exacerbating flooding in the regions.
Heavy rain warnings remain in place for Northland up to the Bay of Islands, Auckland, Great Barrier Island, Coromandel Peninsula, Gisborne and the Eastern Wairarapa and Tararua ranges.
A strong wind warning is also in place for Bay of Plenty, with southeast winds expected to reach severe gales in places, gusting 120km/h.
Cyclone Hale is forecast to move off New Zealand to the southeast late on Wednesday.
Residents near Hikuwai River asked to prepare for evacuation
Gisborne District Council said rain models show the Hikuwai River could peak at 13.5m at 10pm Tuesday - exceeding the 2005 Cyclone Bola level of 12.4m.
Low-lying residents near the river in Uawa have been asked to prepare to evacuate, and farmers encouraged to move their stock to higher ground.
Civil Defence has been activated in all rural and coastal areas.
More than 100mm of rain fell overnight in the ranges and about 40mm in Gisborne. MetService said the region could expect a further 150 to 200mm millimetres of rain on top of what had already fallen.
Unpaved roads are closed, campers near the coast have headed home, and Tairāwhiti Civil Defence is asking people to check on their neighbours.
State Highway 35 will be closed from 7pm Tuesday between Te Puia Springs and Ruatorea after some roads slumped.
Gisborne District Council urged extreme caution on the region's roads after several people were rescued when their vehicle became stuck in a flooded stream.
The council posted on social media that police and the fire service were called to help those trapped in the vehicle after severe weather hit last night.
Unsealed roads will remain closed to freight trucks in the region until 8pm Wednesday.
Tairāwhiti Civil Defence announced the closure, which kicked in at 8pm Tuesday, because of Cyclone Hale.
Forestry company Ernslaw One has backed the decision. Regional manager Bill Wheeler said workers were meant to return from holiday on Tuesday, but that has now been delayed until Thursday.
Meanwhile, contractors are dealing with sewer manholes bubbling due to the large volume of rainfall in the sewer network coming from properties.
Council 4 Waters operation manager Chris Hopman said the emergency valve at the Gladstone Road Bridge had been opened to release the pressure at 8am Tuesday.
"The Wainui Road pump station is at the highest level we've ever seen. We ask residents to please avoid unnecessary water use in the kitchen, bathroom and laundry.
"We need to open the valves into waterways to avoid wastewater overflows into people's homes and through manholes on the street, which can cause health risks."
Flooding and slips on State Highway 1 across upper North Island
Motorists are facing detours on parts of State Highway 1 due to flooding and slips from the heavy rain.
A tree has fallen across the northbound lane of the highway, near Dome Valley in Warkworth.There are no reports of injuries.
The southbound lane is open, but motorists travelling in the area should expect significant delays.
Further north, SH1 over the Brynderwyns is partially closed in both ways due to severe surface flooding and slips.
Traffic is being diverted through the intersection between SH14 and SH1.
In Whangārei, Riverside Drive was down to one lane and smaller cars should avoid Memorial Drive due to surface flooding, police said.
And in the upper North Island, another stretch of SH1 through the Mangamuka Gorge was closed due to multiple large slips and dropouts, Waka Kotahi said.
Motorists were advised to detour via SH10 or delay travel wherever possible.
Flooding in Aotea Great Barrier Island, Hawke's Bay braces for impact
There were also flooding and slips on Aotea / Great Barrier Island.
Local board chair Izzy Fordham said there were a few slips being dealt with by contractors, but campgrounds on the island had already cleared out after last week's storm.
The local emergency management team is on call and Fordham said residents were hunkered down, waiting for the storm to pass.
Hawke's Bay Civil Defence Emergency Management controller Ian Macdonald said his region was expecting heavy rains and strong winds from Tuesday night.
Emergency services were co-ordinating preparations and residents should stay home during the worst of the weather, he said.
Coromandel and Northland: Power cuts and road closures
The rainfall in Coromandel was getting heavier on Tuesday morning, with more set to come.
Thames Valley Civil Defence controller Garry Towler said a total of 400mm of rain was expected by 10pm - the equivalent of the total rainfall over the past week.
He said further road closures could be expected later today.
The state highway on the eastern coast between Hikuai and Whitianga was likely to close, as well as State Highway 25 going into Thames.
Towler said the damage to roads was significant in a one in 20-year storm event.
MetService predicted the severe weather in the region will ease later Tuesday evening.
Several roads across Northland were closed as Cyclone Hale continued to affect the region.
State Highway 15 between Loop Rd, Otaika and Mangatapere, south of Whangārei was closed due to flooding.
Kokupu Block Road was also closed due to flooding, police advising motorists to avoid travel around Northland unless urgent.
Police said two vehicles stuck in flooding on Waiteitei Rd in Wellsford have since made it out. There were no reports of injuries.
A heavy rain warning remained in place for the region till 7pm Tuesday.
More than 100 homes in Coromandel were without power earlier.
"We had quite a lot of impact last night," Towler told Morning Report.
The eastern seaboard has been most affected, he said, and it was highly likely the entire eastern seaboard would close after high tide, he said.
"We've got widespread flooding, we've got a number of slips all the way from Hikuai all the way up to Whitianga. Widespread phone and power outages as well.
"The impact is quite dramatic in terms of what is still to come but also we've got to keep this in context because we had over 400mm of rain last week and there's just no capacity to absorb some of this stuff."
In Whitianga, 136 properties in the town of Whenuakite have been without electricity since 2am. The network company was investigating the cause of the outage.
At 11am, police advised residents to stay indoors and away from the coast as high tide was due at 11.15am.
Several roads in Thames-Coromandel district were closed because of the storm.
