The slip at the Otira Gorge rock shelter, between Arthur's Pass to Otira, deposited about 200cu m of rock on the road.
The highway was completely closed at the slip this morning but had reopened to one lane of traffic by this evening.
Otira Hotel owner Lester Rowntree said things had been "dead" after the slip occurred about 8am.
He was surprised there was a slip when the weather had been so good.
"It's been dry as ... we've not had rain in ages," Mr Rowntree said.
The New Zealand Transport Agency said contractors reported rocks were still falling this morning and about 100m of road guardrail was missing.
Agency journey manager Lee Wright said no detour was available while the pass was closed. Motorists could opt to take the Lewis Pass route.
- By Laura Mills of the Greymouth Star
