The transport agency and KiwiRail say road and rail services remain disrupted in and out of Greymouth by a continually moving slip.
A section of State Highway 7 at Omoto, just outside Greymouth, has been closed since last week after more heavy rain set off the slip.
The landslide first became unstable and required increased monitoring with traffic management from mid-August. The transport agency said the Omoto slip site had been relatively stable since the 1980s, and was easily managed by highway crews with minimal disruptions.
Traffic was now being diverted, and rail services to and from Greymouth had stopped temporarily.
Geotechnical advisors have met this week with highway and rail managers to work out how to return Omoto to a safe highway and rail link.
Transport agency maintenance contract manager Moira Whinham said the movement was caused by groundwater deep within the slip, and the next step would be to see if water from the slip surface could be drained.
"We have a drill rig on its way and from next week will be investigating if it is possible to successfully drain this site.
"By the week starting 11 November, we should have a better idea if the drainage strategy is succeeding and if that will allow us to provide a short-term solution to rebuild the highway and rail track."
UPDATE 1:50PM FRI 25 OCT: SH7 Stillwater to Greymouth at Omoto remains CLOSED.— NZTA Canterbury/WC (@NZTACWC) October 25, 2019
The next update is expected by 10 am Tuesday 12 November.
Detour Route: Via SH7 Stillwater Bridge, via Taylorville to SH6 and Cobden Bridge.https://t.co/ixiEuTwLw4. ^JP pic.twitter.com/Qz1Qjj6CTl
Ms Whinham said wet weather and the saturated ground meant the slope had continued to move, and it had not been safe to do any drilling or drainage work.
Longer term, geotechnical advisors were working on a solution to reduce the potential for such slips, which has had a significant impact on the TranzAlpine service to the West Coast as well as freight.
It has also been inconvenient for locals living east of the slip site, Ms Whinham said.
"We understand the disruption and inconvenience this is causing to people, particularly residents in Kaiata and Dobson, and ask for their patience while we work to make sure we have a robust repair strategy.
"We are working closely with KiwiRail to make sure what we do will provide long term security for both, the state highway and rail link."
KiwiRail's general manager of South Island Operations Jeanine Benson said it was working hard to get the service back as soon as possible, while also developing a long-term remedy for the site at Omoto.