A massive drop-out on State Highway 4 happened so quickly it was lucky no vehicles were swept away by it, says Higgins worker Paul Seconi.
It's his job to guard the ever-expanding drop-out that has closed a lane of the highway just south of Upokongaro.
Mr Seconi said the slip opened up a couple of weeks ago and began as a small crack in the road.
"A truckie noticed a crack in the road about 5am and by 9am this side of the road had gone," Mr Seconi said.
Higgins staff were keeping a round-the-clock watch on the drop-out as it was still moving.
As well, large cracks were appearing in the road near the drop-out.
Mr Seconi said if the drop-out ate further into the road, both lanes would have to close.
The drop-out was most likely caused by weakening of the land due to underground water that came down from nearby hills during floods in June.
Next week construction will begin on a temporary stretch of road, about 100m long, that will be made in farmland next to the road. That would help keep traffic away from the drop-out.
"The farmer will lose a bit of his paddock, but it's only temporary and he'll get it back," Mr Seconi said.
He estimated the road would take about a year to fix.
The debris would have to be cleared and a solid rock retaining wall put in place.
Mr Seconi urged all motorists passing through the area to stick to the 30km/h speed limit.
"People don't slow down. When you get a truck rattling through here you can feel the road shaking, and that's not very good for the slip."