He told Stuff that he was driving from Gisborne to Rotorua at 3pm when he came around a bend and saw the slip across the road.
He, and about three other cars, stopped to see what was going on when the cliff face let go.
"I could see it started cracking underneath - this big crack," he told Stuff.
"Then the whole thing just came down. It's not something you see everyday. It was a bit of an adrenalin rush to be honest."
Contractors are on site clearing the slip, and geotechnical engineers are currently assessing the stability of the cliff face.
Motorists are advised to use State Highway 35 as an alternative route.
The NZ Transport Agency said these alternative routes will add significant time to the journey.