State Highway 4 between Raetihi and the intersection with Fields Track has been closed due to a major crack in the road.
An underslip occurred last weekend, but has become increasingly worse since then, the NZTA said.
- Road to Fox Glacier closed indefinitely due to landslide
- Massive 400m landslide closes West Coast's State Highway 6
- Giant landslide wipes out Fox Glacier access road, again
The road is now unpassable with the massive crack running across it.
"The Transport Agency has commissioned a geotechnical investigation to determine the exact cause of the slip and its severity, and to help identify what repair work needs to be done," NZTA said in a statement.
The road will stay closed until an investigation has concluded.
A detour is in place via State Highway 1 and Fields Track.
Heavy vehicles are advised to use State Highway 1 and State Highway 49.