A massive landslide has been caught on camera as the wild weather ravages South Canterbury, forcing residents to evacuate, closing major roads, and resulting in a district state of emergency.

The video of the slip into the swollen Rangitata River was taken by local woman, Ange Protheroe this afternoon.

State Highway 1 at Rangitata has been closed due to the river level from heavy rain. Photo / via Ange Protheroe video

She said it was taken at the Rangitata diversion intake.

At around 4pm this afternoon, Protheroe and her husband took a drive to see how much water was running down the Rangitata.

They saw brown water racing down the river and hitting into a bank. They spotted some small cliffs give way and they decided to park up and watch it.

Protheroe started filming and managed to capture the large cliffside comedown.

"We were a good 100m away across the river, and the river was really loud, but you could hear the earth cracking before it actually gave way," she told the Herald.

"It was a real rumble. It sounded like thunder as it hit into the water."

After the "unbelievable" scene, the couple looked at each other and said, "That was crazy!"

State Highway 1at Rangitata has been closed due to the river level from heaving rain.

Timaru District Council has declared a state of emergency for the district.

The Upper Rangitata Bridge at Arundel has also been closed, while roads on the West Coast have been blocked by flooding and slips, leaving no road routes between Christchurch and Dunedin until at least tomorrow.

State Highway 6 on the South Island's West coast is closed from Hokitika down to Makarora, with several slips on the road.

Sam Anderson has also posted astonishing drone footage on social media of the rushing Rangitata River at Arundel Bridge.

Timaru District Council, at about 3pm, asked all residents between McLelland, Orion Rangitata Mouth Rds and Rangitata River to evacuate immediately.

Timaru Emergency Operations Centre's Steven Doran said the flooding was causing issues.

"There has been significant rainfall in the headwaters of the Rangitata River that's basically creating a big bulge of water coming down the river. We're expecting that to peak around 7pm tonight," Doran said.

"It's in a rural area so there's not the same sort of risk to property but it has cut off two main arterial routes."

Doran said the water coming down the headlands of the Rangitata River had peaked at 2700 cubic metres per second.

"There is water coming down the river at a level we wouldn't expect to see for at least 20 years between times," Doran said.

"And the river usually runs at about 300 so it's a big, big difference. It's a braided river so it doesn't always flow in the same path as well. So it can change path, and a lot of farmland can be in the way as well."

MetService meteorologist Andy Best said despite the severe flooding in Timaru, the rain had now passed over Canterbury.

"The rain has moved through the Banks Peninsula and is currently on its way further north to Marlborough in the next few hours," Best said.

"There's no rain at all over much of Canterbury now. There's not a cloud in the sky on the satellite imagery."

All the West Coast settlements on State Highway 6, between Hokitika and Makarora, are cut off from each other because of the damage.

Pete Connors, from the NZ Transport Agency, said a huge length of the state highway - more than 350km long is shut because of numerous slips and a bridge washout.

"The real problem we've got is between Fox Glacier and Franz Josef - on what we call the Fox Hills ... there have been some pretty substantial slips," Connors told RNZ.

Connors said it's too early to say how long it will take to repair the roads.

Westland District Mayor Bruce Smith said there had been considerable damage and residents were isolated until the weather improves.

Power is out from Fox Glacier to Paringa, and likely to remain so for a couple of days.