Motorists ran for their lives as thousands of tonnes of rock crashed over State Highway 6, near Queenstown, yesterday in a slip that could block the road for days.

The slip, measuring about 40m across and at least 20m deep, fell on to the highway beneath Nevis Bluff at the western end of the Kawarau Gorge about 10 am, narrowly missing motorists.

The highway - the main road access to Queenstown - is unlikely to reopen until at least Thursday.

A search and rescue team from Queenstown was called to the scene amid fears that people had been buried by the slip.

But police in Cromwell later accounted for the last vehicle seen to pass through the area.

Police closed the highway immediately after the slip, and officers were stationed at the Crown Range turnoff and Goldfields in the Kawarau Gorge to divert traffic.

The slip was witnessed by about 15 people, many of whom got out of their vehicles to watch the rocks fall and were forced to run from the scene when boulders crashed on to the road.

Witnesses said the surrounding hills echoed with a loud rumble as the rocks fell.

The debris tore through a fall-prevention barrier and buried the road, destroying traffic barriers and sending a huge cloud of dust into the sky.

One policeman travelling to the scene saw the cloud from 5km away.

Queenstown man Allister Simpson was driving to Cromwell when he saw several rocks fall on to the road.

He rang emergency services and then watched as some motorists risked their lives by driving beneath the bluff as larger rocks fell seconds before the main slip.

Mr Simpson said it was a miracle no one was killed.

"I saw a few rocks come down and about a dozen cars kept going through, then a major fall came down and everyone just ran," he said.

"People were extremely lucky. It was like a big avalanche. If anyone had been underneath it, they would have stood no chance. They would have been buried."

A group of motorcyclists heading from Queenstown to Wanaka were perhaps the luckiest of all. Three members of the group raced out of the danger zone just seconds before the land gave way, and were caught up in the dust cloud.

The group were separated by the slip, with three motorcyclists making it safely to the Cromwell side.

"They just gunned it. It looked like they were going to get caught but they just made it," said Mr Simpson.

Queenstown Mayor Warren Cooper visited the slip yesterday, ignoring a police barrier to get within metres of the rocks.

He doubted whether the slip would have much effect on Queenstown but said it was important to get the road cleared quickly.

"I hope Transit and Opus get on to it, work diligently and work some long hours to get it done," he said.

"I don't think monsoon buckets will do it. It would take an awful lot of water to move what's up there.

"That's why I'm an advocate for explosives."

The slip is believed to have been caused by warm weather drying and loosening the bluff.