State Highway 6 near Haast is completely blocked after a boulder "the size of a house'' fell onto the road this morning.
The boulder, measuring about 10m by 10m, crashed onto the road about 1km west of Clarke's Bluff, said NZTA senior asset West Coast manager Mark Pinner.
"The rock is about the size of a house, weighing 200 to 300 tonnes. Early indications are that it will not be possible to remove the rock without blasting it into chunks,'' he said.
"Our contractors are already on site assessing stability of the rock face and a specialist driller is heading to the site to begin work as quickly as possible to remove the rock and reopen the highway.''
A specialist geotechnical expert was on standby to fly into the area if required, Mr Pinner said.
The road is currently closed to all traffic, with the next update not expected until 3pm.
The size of the rock meant if the road did reopen, it would likely be down to one lane.
There are no alternative routes, but motorists can call 0800 4 HIGHWAYS 444 449 for updates.