State Highway 6 between Punakaiki and Fox River reopened at 7am today following a mammoth clean-up operation by contractors since the huge slip fell on Sunday afternoon.
New Zealand Transport Agency (NZTA) regional performance manager Pete Connors said the road was open to single-lane traffic.
"There is good line of sight across the area of the slip but drivers need to exercise caution and give way."
By 7am today several vehicles had arrived at the roadblocks either side of the slip waiting to get through.
A freight truck from Greymouth was the first vehicle through from the southern side. The diversion in place via Reefton over the past few days had added considerable time on to the trip for those needing to travel from Greymouth to Westport and vice versa.
Connors said the stretch of road at Meybille Bay would be down to one lane for the next four weeks as a bund had been constructed to protect road users from the "odd loose rock" which could still come down.
The bund has been built from concrete barriers and mounded earth.
Warning signs are in place each side of the affected area as well as signs requiring downhill traffic to give way to vehicles travelling up the road.
Connors said geotechnical experts observed the rock face from a helicopter yesterday before the go-ahead for the road to reopen was given.
They thought the rock fall yesterday morning might have helped by clearing the rock face of loose material as it looked pretty clear, he said.
There would be plenty of tidy-up work happening on the site today following the marathon effort yesterday and thanks were due to those crews.
Connors said there could still be some short delays so drivers should build in extra time for their journeys, particularly today and the rest of the week.
Connors said he thought this was the biggest slip ever in the area and it was certainly the most significant he had seen in the past 20 years.
"It was the size seen at alpine passes such as Otira."
He said a steel rock-fall fence, such as in place further north on the same stretch of road, could not have contained the fall.
"The whole slope itself went rather than fretting off."
Clearing the debris had revealed some damage to the road pavement, however, full assessment would be completed once the road was fully cleaned.
Connors said trucking rocks and earth to the Punakaiki Beach Camp to help with the West Coast Regional Council's sea erosion work there was a fortunate benefit of the slip.
"We harvest rocks from slips all the time to use for our own work. Rock is very expensive particularly in this area."