Strong winds are beginning to buffet the lower half of the North Island as torrential rain starts falling in the South Island.
A rare "red warning" has been issued for parts of the West Coast, with residents told to brace for heavy rain, flooding, slips and possible thunderstorms.
A state of emergency has been declared in the Buller district and an Emergency Operations Centre (EOC) has been activated in response to the severe weather.
They are recommending those in low-lying areas should prepare to evacuate and have a grab bag ready to go.
"If possible go and stay with family and friends on higher ground. Move pets to a safe place and move stock to higher ground. If you have to leave, take your pets with you. If it's not safe for you, it's not safe them."
There have been two slips on State Highway 73 - one at Rocky Point and one at Otira.
The road to and from the West Coast is closed at Arthur's Pass.
High river levels have closed State Highway 8 at Inangahua - with an alternate route via Browns Creek Road.
The road between Hokitika and Haast has surface flooding - so caution is recommended for drivers.
Marlborough is also experiencing surface flooding.
MetService has also issued a spate of weather warnings for other parts of the country – from 140km/h gales in Wellington to heavy rain and strong wind in Taranaki.
Motorists are being warned the wind has already picked up on the Remutaka Hill Rd and the Desert Rd, with those in high-sided vehicles or on motorcycles told to take extra care.
The country's weather service issued its third-ever red warning for Buller and Westland, north of Fox Glacier, from 9pm last night to 6am on Saturday.
The red warning – reserved only for the most impactful weather events – was issued in consultation with Civil Defence and regional authorities.
The rain is expected to be intense and heavy – with up to 380mm in the hills and 200mm around the coast with a peak rate of up to 30mm per hour.
The Insurance Council of New Zealand is warning West Coasters to check their property now for the wild weather starting to hit the region.
"There are some simple things you can do that don't cost anything that will help make sure you and your property stay safe," says chief executive Tim Grafton.
"High winds cause trees to come down on roads, fences, cars and houses which creates havoc. Our infrastructure in many towns and rural areas are unable to cope with the weather we are facing this century, which causes widespread flooding and damage.
"Many West Coasters are either living near rivers or the sea and are used to dealing with heavy rainfall and know what to do. Farmers should secure irrigators and machinery as well as light structures and move property and stock to higher ground" he said.
Overnight weather stations in the region measured more than 140mm rainfall, with MetService warning there was "plenty more to come".
It warned flooding was likely while slips could close roads, disrupting transport routes and isolating communities.
The red warning was prompted by an active front moving across southern and central New Zealand and preceded by heavy rain for western areas.
MetService meteorologist Andy Best said the warning was issued due to the amount and intensity of the rain.
"It is of particular concern for people who are on the roads," he said, urging anyone going away for the school holidays to check the weather conditions before venturing out.
This morning winds across the lower North Island were starting to ramp up with severe northwest gales reaching up to 140km/h expected to batter the capital and 120km/h in southern Wairarapa from this afternoon.
Taranaki would also be hit by powerful gales from tonight.
MetService says the gales are expected to be damaging, bringing down trees and powerlines and sending unsecured structures airborne.
Heavy rainfall warnings were in place for the headwaters of the Canterbury lakes and rivers from the Rakaia river northwards, and between Aoraki/Mt Cook and the Rakaia River, Nelson, west of Motueka, and the western ranges of Marlborough.
Those driving on SH6 Haast Pass, Sh73 Porters Pass and SH60 Takaka Hill were urged to take extra care.