The MetService is warning much of the North Island is in for a wild night of howling winds and heavy rain as the remnants of Cyclone Dovi pass over.
Dovi, which passed over New Caledonia and Vanuatu earlier this week, is expected to cross central New Zealand on Sunday.
MetService says it is a "significant weather event, with widespread adverse conditions likely".
There have already been outbreaks of heavy rain in central New Zealand and severe gale winds are expected over parts of central and northern New Zealand this weekend.
"Significant and hazardous waves with the potential of hazardous cross and rip currents are expected in association with Cyclone Dovi. The largest waves are expected on the west coast of the North Island on Sunday."
Niwa said strong gusts were expected to hit much of the North Island, and winds would be highest close to and around the centre of the low caused by the cyclone.
The winds are expected to be strong enough to cause power cuts and structural damage, Niwa said.
MetService said on its current forecast track, Dovi was expected to make landfall between New Plymouth and Kawhia Harbour during Sunday morning then move eastward across the North Island.
But it said there was still some uncertainty over the exact timing of the cyclone's approach and meteorologists were closely monitoring the situation.
"People are strongly advised to keep up to date with the latest forecasts from MetService, and to follow official advice from authorities."
Utility company Vector says it's preparing for Cyclone Dovi and is advising people to keep their devices charged, and have a power bank and torch handy.
Households whose water supply relies on electricity should make preparations, and people should stay away from downed lines and call 111 if they see any.
Strong wind warning for Auckland
Heavy rain and wind watches are in force for much of the North Island and Waka Kotahi has warned severe winds may force the closure of the Auckland Harbour Bridge tomorrow.
MetService meterologist Peter Little says a strong wind warning will be in place for Auckland from 10pm tonight.
"Initially for Auckland, it's a period of northeast gales which could be gusting at around 100 kilometres an hour," he said.
"But then once the winds turn around to the southwest, which will be Sunday morning and afternoon, there'll be gusts of possibly 130 kilometres an hour. So that certainly has the potential to cause some damage."
Peter Little says the weather system will, however, provide a brief reprieve from the oppressive heat and humidity that has been plaguing residents.
Waka Kotahi says it's likely the Harbour Bridge will be closed tomorrow and has asked Aucklanders to prepare.
Current bridge safety protocols require the closure of the bridge to traffic when sustained wind gusts of 90km/h or higher are likely.
Waka Kotahi National Emergency Response Team leader Mark Owen said they would be monitoring conditions closely, with maintenance crews on standby to react immediately if wind gusts reach the predicted levels.
"Closing the Harbour Bridge is not a decision we take lightly at all, given the resulting disruption and inconvenience for Aucklanders, but we will not compromise on the safety of road users and we won't hesitate to close the bridge if conditions are unsafe for it to be used," he said.
"While a closure may be disruptive, based on the MetService wind predictions it will very likely be necessary."
The transport agency is encouraging drivers of high sided vehicles and motorcyclists to take extra caution on the roads and avoid using the Harbour Bridge entirely until the wind subsides.
Vector has also warned people in parts of Auckland to be prepared to potentially be without power, as it prepares for widespread outages which could take days to resolve. .
Vector's general manager of operations and maintenance Marko Simunac says they have been advised the storm could be the most significant weather event since April 2018, when thousands of homes lost power.
He says the biggest risk time is from tonight until Sunday night, and all of the Auckland region could be affected, but particularly the western side of the city.
Simunac says people are encouraged to ensure they have a torch handy, and phones and other devices charged.
Flooding north of Wellington.
Areas north of Wellington are being hit by heavy rain and flooding this evening.
The Carterton District Council says it's getting lots of calls about flooding, but there's not much it can do.
It advises people to call 111 if their home is flooding.
Fire and Emergency says firefighters have been assisting people with flooding in Masterton, Carterton and South Wairarapa
Several local roads in these districts are shut.
A creek on Upper Plain Road near Masterton has burst its banks, causing flooding to several homes.
Part of State Highway 45 on the Taranaki coastline is shut due to flooding, near Rahoutu, about half an hour south of New Plymouth.
West Coast closure
State Highway 67 - which has 30 slips along a 23-kilometre stretch - will remain closed for another day, as the slips are cleared away. Further heavy rain is forecast to hit the area, however the Buller District Council expects the rivers will hold up.
A council spokesperson said the flood-hit highway was still closed between Karamea and Mokihinui, after heavy rain overnight caused further damage.
Residents were urged to avoid the road and to follow the directions of staff on site.
The highway will be re-assessed tomorrow afternoon.
Crews work on repairing damage from landslips on the West Coast. Photo: Supplied
Heavy rain has also caused significant damage to the Waimangaroa water supply trunk main, along Conns Creek Road.
The council understood the inconvenience of having lost a water supply, the spokesperson said.
"Civil Contractors and crews are working hard to re-establish the pipeline."
Tanker water is available and affected residents can shower at the Pulse Energy Centre.
Taranaki residents told to expect hazardous conditions
Taranaki residents are being urged to prepare for the heavy rain and strong winds expected in the region this weekend.
Between 130 and 220 millimetres of rain is forecast to accumulate about Mt Taranaki.
South Taranaki District Council says surface flooding, slips and wastewater overflows have occured in several locations.
Surface flooding has been identified in areas around Opunake, Te Kiri and just south of Eltham Road on SH 45, Eltham and Kaponga.
Tangahoe Valley Road has been closed due to fallen trees, Morea Valley Road with slips and Waiteika Road has been closed with flooding.
Ararata Road is closed with a slip but the Council says it should be open by tonight.
Wastewater overflow warning signs have been erected at York St Patea, Mana Bay, Patea Boat ramp, Tawhiti Stream Hawera, Middleton Bay and Opunake Lake.
People are warned not to collect fish, shellfish or plant life or come into contact with water at these locations.
Severe south-to south-east gales are expected between 7am and 3pm tomorrow, so driving could be hazardous - especially for high-sided vehicles and motorcycles.
Taranaki Civil Defence is advising residents to only travel if necessary, avoid floodwaters and rivers, and tie down loose objects.