A bridge near Arthur's Pass has been swept away by floodwater.
NZTA have confirmed Goat Creek Bridge on State Highway 73 is down.
Motorist Jess Dempsey said the weather there has been "absolutely terrible".
Dempsey and her husband live in Otira, and discovered the bridge just east of town had been washed out this afternoon.
Another nearby bridge - Kellys Creek - was also flooded, with water covering part of the road.
Goat Creek Bridge was replaced just a few years ago in 2010, after the former timber bridge was deemed to be nearing the end of its life.
A wet and wild weather system has bashed the West Coast, forcing the closure of schools, roads and potentially standing students for the night.
Areas of the coast have been drenched, the Ivory Glacier receiving 391mm of rain - more than Clyde, Cromwell and Alexandra saw in the entire of 2017.
The Hokitika River was threatening to sweep away a campervan parked on one of the braided river's islands.
Westland District Council mayor Bruce Smith said the second alarm for the river had been activated, and there was still another few hundred millimetres of rain forecast.
He says the first alarm is when it hits a level that is considered a flood, the third is extreme and if that goes off they will have some problems "for sure".
Civil Defence has been activated in Westland as heavy rain hammers the West Coast.
The MetService says Hokitika copped more than 800 lightning strikes in a 20-minute period this morning.
Elsewhere, eight schools were closed on the West Coast due to the heavy rain and flooding, the Ministry of Education told the Herald.
Deputy secretary sector enablement and support Katrina Casey said the Ministry is doing what it can to support schools and parents.
"Our local staff have been in contact with the schools and have offered our support where needed," she said.
"The school and road closures affect 900 students.
"We encourage parents to keep in contact with schools and early learning centres tomorrow following today's severe weather."
The conditions have caused parts of roads on the West Coast to crumble and fall away, forcing the NZ Transport Agency to close main highways.
State Highway 6 from Hokitika to Haast and State Highway 73 Arthurs Pass between Arthurs Pass township and Jackson will likely remain closed overnight.
Drivers in the area are being urged by NZTA not to travel unless it is absolutely essential.
"If the road remains closed overnight, a full assessment of the route will be made at first light tomorrow," NZTA Regional Systems Manager Pete Connors said.
"It is still raining steadily in the region, and there is widespread surface flooding and several slips that our crews have been unable access.
"The situation is being constantly assessed, and our primary consideration is the safety of road users, as well as the safety of crews working to clear slips and unblock culverts."
MetService meteorologist Gerrit Keyser said at about 11am the highest rainfall totals for the past 12 hours were in Arthur's Pass with 112mm, Mt Cook Village with 102mm, Franz Josef with 81.5mm and Milford with 75mm.
Strong winds were also starting to ramp up about the Canterbury high country, Wellington and southern Wairarapa. Gusts could reach 150km/h in exposed parts of Canterbury, and 130km/h in other affected areas.
The strongest winds recorded so far were 80km/h at a station near Lumsden in Southland, and 65km/h at Le Bons Bay on Banks Peninsula.
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