A short, sharp burst of wild weather has caused havoc on the South Island's West Coast, trapping motorists as slips blocked State Highway 6, even causing a damaging tornado on the northern outskirts of Greymouth.

SH6 between Ross and Haast was closed just after 3pm yesterday after heavy rain brought down trees and slips along the route. It has now fully reopened.

Road crews have been out since first light this morning to reopen the highway. However, multiple works sites along the route will have one lane and speed restrictions, particularly at the larger slip site near Lake Ianthe.

Waka Kotahi said some delays can be expected, and road users should factor this into their travel times, urging drivers to take care and drive with caution.

Further inspections and assessment of damage to SH6 will be carried out after river levels along the lower West Coast recede, the agency said.

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“Power contractors are working to restore power to the areas around Franz Josef, Fox glacier and Haast. There has also been issues with the communication network.

"Those stranded in Whataroa overnight were looked after by locals but have now been able to leave the area.”

Drivers intending to travel on the West Coast are urged to check road conditions before they travel. The latest updates can be found on Waka Kotahi's website.

Routes from Ross to Whataroa, Franz Josef to Haast and Whataroa to Franz Joseph are now open but a 30km/h speed restriction is in place and delays are expected.