A large slip has closed State highway 6 at Meybille Bay, north of Punakaiki.
The New Zealand Transport Agency said last night the Coast Road would be closed until further notice.
The slip occurred in a narrow area of bluffs that has been prone to landslips in the past.
"A slip this morning covered at least one lane but traffic was able to pass safely until after 3pm, when more material gave way. More has fallen since then also," NZTA regional performance manager Pete Connors said.
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NZTA contracting teams will be working the slip with diggers from both sides, with sluicing to remove loose material and rocks this morning.
The next update was likely to be this afternoon, Mr Connors said.
Motorists were advised to take the inland route, State highway 7, via Reefton.
Road blocks are in place south of Westport, at the intersection of State highways 6 and 7, and at Rapahoe, just north of Greymouth.
- Greymouth Star