Heavy rain in the South Island today has wiped out a road to an Arthur's Pass bridge, and a search is underway for a person missing in Haupiri River.

Lester Rowntree from the Otira Hotel said the road leading onto the Goat Creek Bridge to the south of Otira had completely collapsed.

"The approach to the Goat Creek Bridge is completely gone so there's about a 20 metre long hole in the road."

A Facebook image shows the approach to the bridge to Arthur's Pass collapsed in the severe weather, with a power pole near beside it also nearly falling.

Mr Rowntree said he got a call from a neighbour that there was a problem at the bridge and when he got there, there was just a small hole in the side of the road.

"So I put a tape across because I had a bit of a look under where that gap was and the whole road was completely undermined and ready to collapse into the river, and just as I got the tape across a truck came along and was about to go over it so I stopped him and just as we were having a look it just started all falling into the river."

He said it could have been a major catastrophe if a bus or car had gone over half an hour before when the road gave way.

"We were very lucky we didn't lose anyone today."

Mr Rowntree said it would be a big repair job to fix the bridge. He believed State Highway 73 could be closed for days meaning people wanting to travel between Canterbury and the West Coast would have to take an alternative route.

Photo: Facebook / Greymouth i-SITE

At Haupiri River, emergency services and members of the public are searching for a missing person.

Greymouth Police said emergency services were notified just before 8pm.

Meanwhile, the heavy rain has caused a creek in Mount Aspiring National Park to rise and stranded 26 Otago Boys' High School students.

In a long-standing tradition, the school takes its Year 9 and 10 students into the Matukituki Valley each year to get to know each other away from technology.

The class of 26 boys was due back today, but the hut they are staying in was cut off by the rain.

Deputy rector Mark Hooper said the boys were safe, dry and in high spirits with plenty of supplies.

️ Extreme rainfall totals in the South Island ️



Ivory Glacier 391 mm

[more than Clyde, Cromwell & Alexandra saw in the entire year of 2017!]

Mt Philistine 294 mm

Mt Cook 212 mm pic.twitter.com/SH5rPuNOTI — NIWA Weather (@NiwaWeather) November 8, 2018

The group will come out of the valley tomorrow, and go home to Dunedin, if the weather permits.

The Otago high school students aren't the only ones facing difficulties with Harihari's South Westland Area School students being escorted home due to the weather conditions.

Earlier today, the highway from Hari Hari to Franz Josef was closed due to flooding.

Civil Defence has also opened a welfare centre for locals and tourists in Hari Hari and have other centres on standby.

The latest transport agency update said heavy rain in the area had shut a section of State Highway 6 between Fox Glacier and Hokitika, while SH73 through Arthurs Pass is also closed.

It is understood the highways might be closed overnight as crews work to clear slips and debris.

There were reports of water across the road in South Westland, and more heavy rain was forecast.

Authorities are preparing for possible flooding in Hokitika as heavy rain makes its way up the West Coast.

Westland Civil Defence acting controller Simon Bastion said 200-300 mm of rain was forecast over the next 24 hours.

Accommodation providers in Hokitika were packed to the rafters as travellers search for last-minute refuge from the deluge.

Mountain Jade Backpackers owner John Wilkinson said they were fully booked and have had to turn away 20 to 30 people who had nowhere to go.

A lot of those staying at the backpackers would have been camping in tents, he said.

Area warnings are also in place for State Highway 60 from Riwaka to Takaka due to forecast rain from now until tomorrow morning.

Strong wind warnings are in effect for Canterbury, including the alpine passes. Parts of Otago and Southland are also braced for heavy rain.

Further north, strong winds have caused some flights into Wellington Airport to be delayed.

Westland Mayor Bruce Smith said the worst of the rain had passed over causing trees and even a van to float down river.

Mr Smith said Hokitika township had escaped the flooding - but the river was at very high levels.

"[The campervan is] not parked there now - it might be heading to Aussie across the Tasman - and my instinct is that the weather event has passed over the top of us and I think the worst is over," he said.

Mr Smith said locals stayed inside with their gumboots at the ready and tourists have sought shelter at the pub.

"The tourists were just saying look head for the pub - go and have a meal, have a beer and sit by the fire," he said.

Mr Smith said the clean-up would probably begin later today.

Meanwhile, the temperature in North Canterbury soared over 30 degrees today.