Impatient motorists are trying to remove road cones to get through the blocked Ngauranga Gorge in Wellington as traffic reaches a snail's pace in the evening commute.

A large slip on State Highway 1 at the gorge has blocked all three lanes heading into Wellington.

The slip struck just as the city was preparing to host the parade to honour Team NZ winning the America's Cup.

All three southbound lanes and two northbound lanes of the gorge were affected by the rockfall.

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"We've had a few people who have tried to move road cones and try to get through traffic managements, which is not helpful because then they're going to come up against people who are doing important work to try and get the road open," said New Zealand Transport Agency systems manager Mark Owen.

"Please, if the road cones are there, they're there for a reason and it's for the safety of all the road users and also the road workers as well."

Dramatic dashcam footage captured by motorist Asispal Sandhu shows the moment the slip hit the highway.

Cars were already braking on the busy gorge road as debris started to tumble down the rockface and spill onto the road.

The footage was published on the Wellington - Live Facebook page.

#WATCH The moment the landslide all came down on SH1! The road is currently BLOCKED! Video by Asispal Sandhu

Posted by Wellington - LIVE on Monday, 10 July 2017

Police are warning motorists of long delays, saying the rockface will need to be stabilised and the debris removed.

Motorists should avoid SH1, and instead head into the capital via Johnsonville or by taking SH2 from the Hutt Valley.

Patients with hospital appointments tomorrow should plan their journey in advance, Capital and Coast DHB advised this afternoon.

Around 30 people were unable to attend their appointments at the hospital today, due to the gorge closure.

It is still important to attend booked appointments, including dialysis, X-ray, cancer treatment, and other specialist appointments, a spokesman said.

The free shuttle between Wellington Regional Hospital and Kenepuru Community Hospital will continue to run, but will take an alternative route.

Anyone planning to drive to their appointment should plan their route in advance, and allow more travel time.

There are hundreds of hospital appointments booked for tomorrow.

"Our booking centre staff are doing their best to contact people living in the Northern Suburbs, Porirua and Kapiti to talk about travel arrangements."

The slip comes some months after one in the Manawatu Gorge, which has closed a major road indefinitely.

"I don't think this is in the same category as what has been happening in the Manawatu Gorge, which has been problematical for some time," said professor Michael Pender of the University of Auckland's department of civil and environmental engineering.

"I have had a long interest in the greywacke slopes around Wellington and been impressed how stable they have remained, even in the recent earthquakes. If Wellington has had as much rain in the last few months as Auckland, then it might be a problem with a build-up in water. It is difficult to say much more without actually visiting the site.

"I expect the first priority is to clear the debris, then geotechnical engineers and engineering geologists will do a thorough assessment of the situation and decide what, if any, stabilisation measures are necessary."