A helicopter was used to help clear the Ngaio Gorge Rd slip in early August.

Another 100 tonnes of rocks and rubble have come down in Wellington's Ngaio Gorge, further exacerbating a land slip blocking the road through the area.

More debris fell on Thursday afternoon while crews were working to clear the busy road that links Wellington's central city to the northern suburbs.

People at the scene reported seeing "tonnes and tonnes" of rock come crashing down.

ROSS GIBLIN/STUFF Wellington mayor Justin Lester and Downer's Crispin Beek check out the Ngaio Gorge slip, which has completely blocked the road (file photo).

Wellington City Council spokeswoman Victoria Barton-Chapple said the digger working in the area had forewarning from monitors in place, and was out of range 12 minutes before any materials fell.

READ MORE: Work to clear Wellington's Ngaio Gorge slip aided by scientific expertise

"It's a complicated job, but we're working closely with the Geotech crew, monitoring systems are in place, and we are now reassessing the plan so we can move material from a new angle," she said.

MONIQUE FORD / STUFF A helicopter drops water to clear a large slip on Ngaio Gorge Rd, Wellington, on Monday evening.

On Tuesday, helicopter sluicing removed loose rock from the site with the aim of eventually bringing in a 20-tonne excavator – a plan of attack that Wellington City Council staff said was dependent on safety.

GNS Science staff visited the site last week with engineers from Opus consultants to collect information for their own research on modified slopes in Wellington.

As part of that research, the team used a laser scanner to build a detailed 3D model of the landslide to determine the volume of rock involved in the landslide and help engineers assess how best to remove the debris safely and stabilise the slope.