The New Zealand Transport Agency has announced $1.5 million of work above the vertigo-inducing slip beside State Highway 6 at Knights Point on the West Coast.
It warned it could not rule out the potential loss of the road, a major tourist route and lifeline to Haast.
The huge slip, which has started eroding, occurred at the Epitaph Cutting, 27km north of Haast, during heavy rain in October 2012, leaving a 300m drop into the sea.
It slumped by 20mm in mid-December 2014 and dropped a further 5mm two weeks later.
NZTA said yesterday it had a $1.5 million work programme planned as a result of a recent geotechnical review.
It also revealed "minor slumping'' had affected more than 50m of the road to the north of the slip.
The Haast highway was cut into the high bluffs above the Tasman Sea in the early 1960s. No alternative route exists.
NZTA regional performance manager Pete Connors said erosion was occurring to the slip face itself, below the road.
There is a 5m separation between the top of the slip and the edge of the road, and Mr Connors said that remained relatively unchanged since October 2012. However, the erosion would eventually affect the road and would need to be managed through engineering.
"At present, this is not considered to represent an immediate risk to road users,'' Mr Connors said, adding in such a volatile environment, the loss of the road "can never be ruled out''.
"For this reason, the transport agency will undertake further geological investigation of the wider area beyond the slip itself to determine possible longer-term roading options should they become necessary.''
A larger geological process had been identified, resulting in ongoing minor road slumping, affecting a 50 to 60m section of road to the north and immediately adjacent to the main slip.
This area was believed to have been moving slowly prior to the other slip, in late 2012.
Monitoring stations had recorded about 50mm of "settlement'' in the past four years, triggered by heavy rain.
"We expect to see small amounts of slumping continuing. However, this does not mean that there is an immediate risk to the highway,'' he said.
"This could be ongoing for several years before the site settles down.''
This type of movement had been observed in several locations in South Westland.
"It is unlikely, but possible, that a major movement event could occur, which could lead to a temporary road closure.
"If this occurred, it is expected that the road would be closed for up to three days while a temporary route is constructed through the site.''
An earlier option to cut into the hillside above the road had been ruled out given the potential for that to trigger greater instability.
- By Laura Mills of the Greymouth Star
