A helicopter has been brought in to help clear a major rock slip on Ngaio Gorge Rd in Wellington.

The site on the busy Wellington route was assessed on Monday after an estimated 4000 cubic metres of rock came down since Saturday morning.

"We were thinking of getting the 20-tonne digger up to the top and clearing the rock from up there, but we've realised it's unsafe for the digger at this stage," Wellington City Council transport assets manager Deven Singh said on Monday afternoon.

ROSS GIBLIN/STUFF Wellington Mayor Justin Lester and Downer's Crispin Beek on Monday, checking out the Ngaio Gorge slip that has completely blocked the road.

"So we've changed our plans. We're getting a helicopter in to sluice the material. It's going to drop some water in from the top to get rid of as much of the loose material as it can.

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"Once that's done, we'll then decide how to address the rest of it."'

KEVIN STENT/STUFF A large slip on Ngaio Gorge Road in Wellington is expected to keep the busy route closed for much of the week.

The helicopter was due to begin work at 4.30pm on Monday.

It was a "wait and see" situation as to how much more rock came down during the sluicing and how much remained, Singh said.

The council would take advice from geotechnical engineers on how stable the site was after it was flushed out.

MONIQUE FORD/ STUFF Lester visited the site on Monday and said the road would be closed for at least a week, and possibly two.

Wellington Mayor Justin Lester visited the site on Monday and said the road would be closed for at least a week, and possibly two.

About 4000 cubic metres had fallen during 20 slips at the site since 3am on Saturday.

Some vulnerable rock on the hillside would be brought down by workers to ensure it did not pose a risk to motorists when the road reopened, Lester said.

KEVIN STENT/STUFF Ngaio Gorge road in Wellington is completely blocked off after a slip that began early on Saturday morning became much worse on Saturday night.

"We're working as quickly as possible, but it's a very big slip, and people are going to have to be patient."

Detours were in place and security staff were put on site to stop people getting too close, a council spokesman said.

Old Porirua Rd has been closed to traffic but residents and cyclists will be allowed downhill access.

Motorists heading into the city on Monday morning reported alternative routes from the northern suburbs being considerably busier and slower than usual.

Motorists have been advised to take care and reduce speeds on those routes, including Onslow Rd and Wadestown Rd.

Meanwhile, 13 people remain displaced from a two-unit property in Devon St, Kelburn, after a slip on Thursday.

It has been confirmed a broken pipe sticking out from the two-unit property was a sewer main, which has been temporarily reconnected to the city's wastewater system.

The situation was not deemed a public health issue, a Wellington City Council spokeswoman said.

Residents will be allowed back into the property following engineer approval, expected to be completed on Tuesday.