State Highway 6 is closed between Haast and Fox Glacier due to a slip as the West Coast braces for a major deluge.
Waka Kotahi said the route would remain closed overnight and be reassessed in the morning.
There is no detour available so motorists are asked to avoid the area or delay their travels.
West Coast Emergency Management began pre-positioning staff and resources this afternoon ahead of a forecasted torrent expected to hit the south and west of the country.
MetService has placed the West Coast under an orange heavy rain warning until Thursday night with up to 800 millimetres expected to accumulate in places.
The area is set to bear the brunt of the deluge, with 62 hours of rain forecast.
The region's group manager, Claire Brown, said staff were constantly getting updates on the situation.
"We're starting to pre-position resources around the district so that we know we've got the people in the right place," she said.
"There are some other resources in terms of generators located and we're also working closely with our critical infrastructure agencies, such as Chorus and the Transport Agency (NZTA), ensuring they are making their own preparations in the event that there is any interruption or impact on their services."
Staff on the ground were giving regular updates of the conditions where they were, as well as monitoring river levels and the latest forecasts, Brown said.
She was based in Greymouth and said rain was yet to fall there, though the sky was "grey and heavy".
"We understand the rain has already begun down in the Westland District and is already having some effect on the river catchment, so we're expecting that to continue over tonight and into tomorrow," Brown said.
"It's pretty persistent [now until Thursday]. We're hoping that it might let up a little bit into Wednesday evening but certainly between now and then the rain will be heavy."
The forecast rain was a huge amount, even for the West Coast, she said.
"We're accustomed to large amounts of rainfall, that's the nature of the environment we live in."
"This is happening over a number of days, over quite a large area, so it's really difficult to pinpoint any particular area or time [of concern].
"We're encouraging everyone to keep up to date with the weather forecast and before you leave home, check the roading conditions, check that everything is okay where you're travelling," she said.
People should also have some emergency supplies in their vehicle if travelling through or around the region in case they got caught out or stuck somewhere unexpected.
This should include warm clothes, some food and water, Brown said, and it was good practice to always carry it.
"Up to 800mm of rain over that period of time is a significant amount of rain, so we're asking people to take every precaution."
There is also a strong wind watch for Fiordland over the next nine hours, and the Canterbury High Country for the next 26 hours till 2pm on Wednesday.
Rain already falling
MetService said more than 100mm of rain has already fallen in Milford Sound.
Forecaster Gerard Bellam said the front had hit the Fiordland and was slowly beginning to move into Westland.
"We've seen that rain set in this morning with heavy falls registered - already we've had 117mm recorded at Milford Sound, so certainly, the train is rolling through now," he said.
Rapid lake and river level rises were expected, Bellam said.
"We're going to see quite a lot of spill over from the West Coast into those Otago and Canterbury headwaters.
"People need to be alert and keep up to date with the latest warnings and watches. Rivers in the eastern parts of the South Island will certainly be rising rapidly," he said.
Wind gusts of 179 kmh had also been recorded in exposed, high altitude areas in the Canterbury High Country, Bellam said.
The heavy falls were set to stick around for most of the week because a large high sitting out near the Chatham Islands was blocking the front's normal progression across the country, he said.
Great walks closed
The Department of Conservation had closed several Great Walks and tramping tracks in the South Island as heavy rain bore down.
In Fiordland, the Milford and Routeburn tracks were closed, and anyone on them would remain at the huts with rangers until they reopened.
The Hooker Valley Track in Aoraki Mount Cook National Park was also shut until the weather had cleared and staff could check the swing bridge and track.
"Four-wheel-drive access into the Ball had also been closed," Western South Island Regional Operations Director Mark Davies said.
Visitors booked to stay in those huts between today and Saturday had been contacted and had their booking refunded, he said.
On the West Coast, the Paparoa Track was set to close between the Smoko Carpark and Pororari hut overnight on Thursday.
The Kepler Track remained open but people could request a refund if they wanted, Davies said.
"NIWA provides DOC with 12, 24, 36 and 48-hour updates about severe weather," he said.
"With an impending severe weather event - such as that forecast for this week - Visitor Centre staff use the DOC booking system to notify booked visitors 24-48 hours ahead of the start of their trip so they know tracks are closing.
"There are also closure alerts on individual track pages on the DOC website," Davies said.
Closures would be reassessed on Wednesday.
Other areas affected
While there are no warnings in place for Dunedin, MetService is forecasting rain with some heavy falls for the city on Thursday.
The Queenstown Lakes District Council said contractors were inspecting vulnerable sites, and it was working with emergency services and partner agencies.
Otago Regional Council says it has flood duty staff and volunteer response groups based across the district.
About 200 to 300mm of rainfall is expected about the main divide, and 100 to 150mm within 15km further east. Heavy rain is expected to return later on Wednesday and become more widespread through Otago.
Niwa is warning that flooding "is a concern" for the hardest-hit areas, and the National Emergency Management Agency says that in times of crisis or civil defence emergency, a battery-powered radio or a car radio remain essential lifelines if power is out and other forms of communication are unavailable.
NZ Transport Agency Waka Kotahi (NZTA) is urging motorists to plan ahead and check the latest conditions before driving on the West Coast.
"Early forecast rainfall totals in the region are significant, and may impact parts of the state highway network, including the possibility of surface flooding and debris on road surfaces.
"NZTA is monitoring the conditions closely, receiving regular updates from MetService and working closely with local authorities. Given the severity of the forecast weather, state highway closures are possible, but crews remain ready to respond."
Heavy Rain Warning - Orange
Impact: Heavy rain may cause streams and rivers to rise rapidly. Surface flooding and slips are also possible and driving conditions may be hazardous.
Area: Westland District south of Hokitika
Period: 62hrs from 10am Tue, 9 Apr - midnight Thu, 11 Apr
Forecast: Periods of heavy rain. Expect 600 to 800mm to accumulate about the ranges, and 200 to 300mm about the coast. Peak rates of 25 to 35 mm/h about the ranges from mid Tuesday afternoon until Wednesday evening, then again from mid Thursday morning onwards. This is a significant heavy rain event with the potential for upgrade to a Red Warning.
Area: Headwaters of Canterbury Lakes and Rivers south of Arthur's Pass
Period: 51hrs from 2pm Tue, 9 Apr - 5pm Thu, 11 Apr
Forecast: Periods of heavy rain. Expect 400 to 600mm about the main divide, and 200 to 300mm within 15km further east, especially south of the Rakaia River. Peak rates of 15 to 25 mm/h.
Area: Headwaters of the Otago Lakes and Rivers
Period: 22hrs from 10am Tue, 9 Apr - 8am Wed, 10 Apr
Forecast: Expect 200 to 300mm about the main divide, and 100 to 200mm within 15km further east. Peak rates of 15 to 25 mm/h. Heavy rain is likely to return later on Wednesday, with widespread rain through parts of Otago. A further Warning or Watch may be issued closer to the time.
Area: Fiordland about and north of George Sound
Period: 18hrs from 9am Tue, 9 Apr - 3am Wed, 10 Apr
Forecast: On top of what has already fallen, expect another 80 to 110 mm of rain. Peak rainfall rates of 15 to 25 mm/h. Heavy rain is likely to return on Wednesday afternoon and a further Watch or Warning is likely to be issued closer to the time.
- RNZ/ODT Online
