A significant slip is on the move in Central Otago and could potentially close the road overnight.
The slip is about 30 metres long, 20 metres wide and 15 metres deep, and is near the intersection of State Highway 8 and State Highway 8B near the town of Cromwell.
NZTA says anyone who is travelling to Clyde, Alexandra or back to Cromwell on Tuesday should leave this afternoon and not wait until the evening to drive.
"We may need to close this highway overnight from 5pm for everyone's safety. We will also have short delays this afternoon if we are sluicing," NZTA maintenance contract manager Mark Stewart says.
It will confirm on Tuesday afternoon whether SH8 to Alexandra will close overnight. There are no other easy detour-linking roads between Cromwell and Alexandra.
Crews have reduced SH8 to a single lane using manual traffic management for about 100 metres in both directions from the SH8B junction.
The slip was first noticed moving relatively slowly but steadily on Tuesday morning and traffic was reduced to a single lane as a result, Stewart says. NZTA is planning to sluice the area using a helicopter once weather allows. The highway will close temporarily while this is underway.
"A split in the land has developed above the highway and has destabilised the material underneath it," Stewart says.
"We thank people for building in extra time if they are travelling south on SH8 through the Cromwell Gorge, towards Clyde and Alexandra and to bear in mind the highway could be closed overnight."