• Thunderstorms are expected in Fiordland, Westland, Buller, Nelson, Marlborough and Mt Taranaki.
• It's going to be windy in Wellington.
• Heavy rain is expected over the rest of the country.
Hokitika residents have been hit with flooding, while thunder and lightning have been recorded in the Milford Sound.
Westland was in civil defence mode this morning as heavy rain continued to batter the district.
There was substantial rain overnight in Hokitika and South Westland with hourly rainfalls exceeding 24mm per hour around midnight.
By 7am, Hokitika had received 200mm of rain since 10pm yesterday. A further 220mm of rain was expected by tomorrow afternoon.
At 10am, the Westland District Council moved into civil defence readiness. Adding to the heavy surface flooding which closed parts of Sewell, Weld, Rolleston, Davie and Jollie streets was a burst water main in Stafford Street.
West Coast Regional Council hydrologist Stefan Beaumont said by 10am the Hokitika River was through its first stage alarm, and at Franz Josef, the Waiho was going through its second.
However, the rivers were starting to plateau.
Further south, the Jackson Bay Road - closed by multiple slips caused by heavy rain three weeks ago - was also closed again. Access had been restored only four days ago.
Parts of State highway 6 were also hit, with the New Zealand Transport Agency reporting slips at Ross and Fox Glacier.
Part of the Ross Cemetery hill came down in the rain and closed the road temporarily.
A small slip also blocked one lane at Fox Hills.
Just before midday, NZTA said its road crews were working hard to keep the highway south of Hokitika to the glaciers open as rain continued to "bucket down".
Drivers were advised to anticipate delays and take extreme care, NZTA journey manager Lee Wright said.
The extreme weather was a result of a front moving up the country, MetService meteorologist Lisa Murray said.
Rain is expected to ease in Westland for a period this afternoon and early evening, but more heavy rain is likely Friday morning.
Westland and the top of the South Island might also experience thunderstorms, she said.
Weather system to bring thunderstorms
The MetService was this morning warning that a nasty weather system rolling in from the Tasman Sea today has large swathes of New Zealand in its sights, as it drags a series of fronts bringing heavy rain and thunderstorms on to the country.
It'll be a drenching for some, but best to leave the brollies at home -- gale-force winds are also likely for some.
The MetService updated heavy rain warnings this morning to include the ranges of the eastern Bay of Plenty, Taupo and Taihape. They had last night warned the north and west of the South Island, from Nelson and Marlborough down the west coast to Fiordland, as well as Mt Taranaki and the central North Island high country, to expect heavy rain.
Barring the central North Island high country, those areas could also expect thunderstorms through to tomorrow morning, the MetService warned.
The rainmaker is the result of a series of Tasman Sea fronts moving over the country from today until Friday.
"The heaviest falls are expected to be about Westland, where another 300mm could accumulate about the ranges, on top of what has already fallen."
Coastal Westland could expect 100mm to 150mm today, with more heavy rain tomorrow. The ranges of Nelson, the Richmond Range and Mt Taranaki were likely to receive 200mm, with 100mm expected in coastal Nelson. Up to 200mm could also fall over 36 hours from this evening in the central North Island high country, they warned.
"This is a significant amount of rain for these areas, and people are advised that streams and rivers will rise rapidly, slips and surface flooding are also possible."
A watch has also been issued for severe gale-force winds in parts of Canterbury, Nelson, Marlborough, Wellington, Taranaki and Whanganui from this afternoon until tomorrow morning.
Squally rain bands sprinkled Auckland this morning, and showers were also expected in Hamilton, Tauranga and Wellington.
WeatherWatch head analyst Philip Duncan said social media was "lighting up" as those desperate for rain got it.
"Some of the rain will be locally very heavy, fuelled by moisture rich subtropical air and slowed down by our hills and ranges but also a very large blocking high to the east of the country.
"It's possible downpours in the west and northwest may cause localised flooding."
Meanwhile, Cyclone Winston has re-intensified back to a severe, category three, cyclone. It has been tracking northeast towards Tonga, after being pushed away from New Zealand by a large high.
However, tomorrow it is likely to do a U-turn and begin tracking back the way it came, Mr Duncan said.
"Long-range models are still conflicted about what happens to Winston once it finally starts to properly drop south. Today reliable models show the cyclone tracking back towards New Zealand to our northeast around February 23 onwards."
Models showed it dropping south and weakening well before reaching the North Island or coming further on to the North Island, he said.
"We'll certainly need a couple more days to lock in the future path of this cyclone with a mind of its own."
Weather caused traffic disruption
Today's weather caused disruption in Auckland this morning.
Auckland Transport spokesman Mark Hannan could not be contacted, but the council-controlled organisation tweeted just after 8.30am that Gulf Harbour ferries had been cancelled because of the weather and ferry services would be operated by bus until further notice
Road and rail commuters also faced headaches getting to work this morning.
A four-minute power outage between Westfield and Wiri at 7.32am caused delays and cancellations on the Eastern, Onehunga and Southern lines.
And there were slow journeys on almost all the city's motorways. Breakdowns, now cleared, on State Highway 1 and State Highway 20A, the main road to Auckland International Airport, snarled up morning rush-hour traffic.
Traffic was also at a crawl in the west.
A motorist travelling to the city from West Auckland described an agonisingly slow journey just before 9.30am.
"I'm in West Harbour and it took 20 minutes to get on to the Royal Rd on-ramp and it's taken an hour from Royal Rd to coming up to Newton now."
A mid-morning crash also caused delays when it blocked the right city-bound lane on the Southern Motorway just before Takanini, and a serious crash also closed Beach Rd in Browns Bay before 9am. Police confirmed a person was critically injured.
In southeast Auckland, three people were hurt when a van and a car collided on the Clevedon-Kawakawa Rd at Kawakawa Bay.
The New Zealand Herald understands the 8.30am crash involved a mail delivery van and a car, and mail lay scattered over the road.
Inspector Barry Smalley, of Auckland, said one person with moderate to serious injuries was trapped following the crash and was being extracted by emergency services. Two others had less serious injuries.
- Additional reporting: Hokitika Guardian