Heavy rain on the West Coast has caused flooding on some roads, and MetService is warning there is worse to come.
The weather agency has warned people in the middle of the country to avoid travelling, with gale-force winds and heavy rain forecast for the South Island and lower North Island.
The main issue for the West Coast was the persistent nature of the rain, which might bring 400mm to the region's western ranges, it said.
The rain caused surface flooding on Wednesday on State Highway 6 from Mokihinui to Fairdown and Hokitika to Haast, SH73 from Otira to Harris Creek and SH7 Kaiata to Greymouth.
Strong winds also affected some roads in the South Island, particularly SH73 Springfield to Arthur's Pass, SH7 Waipara to Springs Junction, and SH6 Hokitika to Haast.
The New Zealand Transport Agency said the winds would make driving conditions hazardous. It would have contractors on standby on both sides of Rimutaka Hill if the road needed to be closed.
Gusts of up to 150km/h were forecast from Wednesday in exposed places in Canterbury north of Ashburton, Marlborough and the Wairarapa, with winds of up to 160km/h expected in Wellington.
Gale warnings were also in place for Clutha, Fiordland, Southland and Stewart Island.
Wellington braces for 'sudden strong gusts'
The capital has so far escaped the worst of the bad weather.
Planes, trains and Cook Strait ferries were still operating late on Wednesday, and trolley buses were able to stay in service, despite winds of up to 122km/h.
However, MetService said the wind was expected to strengthen overnight in Wellington and Wairarapa.
It said the worst rain had hit the West Coast and the Southern Alps, with heavy falls at Arthur's Pass, Franz Josef, and the Hokitika Gorge.
MetService communications meteorologist Lisa Murray said the weather would be at its peak late on Wednesday and in the morning on Thursday.
"There will be some really strong gusts around then. Particular in high-sided vehicles or motorbikes or even - if you are brave enough - bicycles, which I wouldn't recommend, take care and drive to the conditions because these will be really sudden strong gusts."
Surface flooding for West Coast
The Westland District was expected to have the most rainfall - with as much as 300-400mm falling in the ranges over 24 hours.
MetService's monitors earlier showed a large band of heavy rain rapidly approaching Hokitika.
The agency said up to 40mm of rain an hour could fall in Westland at the peak of the storm.
It said the weather "bomb low" could cause flooding, sudden river rises and slips. The winds could bring down trees and powerlines.
Ms Murray said the rain in Westland was expected to be heaviest on Wednesday afternoon.
Metservice forecasters said 100mm of rain could fall tonight and tomorrow morning in Buller and the ranges of northwest Nelson and Nelson Lakes.
What is a 'bomb low'?
While the term "weather bomb" is often used colloquially, MetService said New Zealand experienced few actual "bomb lows".
The technical term for a bomb low is "explosive cyclogenesis".
A bomb low is a weather system in which pressure drops by at least 24 hectopascals in 24 hours.
Pressure can be read by looking at isobars (the curved lines on a weather map) - the numbers marked on each line show the hectopascals along that ridge of pressure.
As the low moves off the country to the east on Thursday it will bring unseasonably cold overnight temperatures, which could bring snow overnight Wednesday through to Thursday.
The storm is likely to bypass Auckland and the upper North Island where summer highs are set to continue.