Timaru residents have been issued an evacuation notice after one-in-20-year flooding of the Rangitata River blocked off State Highway 1.

The Timaru District Council has asked all residents between McLelland, Orion Rangitata Mouth Roads and Rangitata River to evacuate immediately.

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The evacuation notice appeared on the Timaru District Council Facebook page shortly after 3pm today.

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The flooding near Timaru, and also on the West Coast, has blocked off all 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.

Earlier today, a local state of emergency was declared in Timaru following ongoing heavy rainfall.

The flooding has closed the State Highway 1 bridge over the river, as well as the back-up Arundel Bridge.

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

"There's 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.

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"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 ."

Extensive rain and strong winds have been battering the West Coast. Photo / Facebook
Extensive rain and strong winds have been battering the West Coast. Photo / Facebook

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."

Part of Little Man Bridge on State Highway 6 near Te Taho has been washed away. Photo / Facebook
Part of Little Man Bridge on State Highway 6 near Te Taho has been washed away. Photo / Facebook

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 Transport Agency, said a huge length of the state highway - more than 350 kilometres - 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's 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.