A large rockfall near Mount Aspiring has closed State Highway 6 south of Makarora until at least midday tomorrow.
NZ Transport Agency senior network manager John Jarvis said the 2000 cu m rockfall came down at 6am this morning about 15km south of Makarora.
The rockfall was roughly 50 metres long and 4-6m deep.
"We currently have a helicopter using a monsoon bucket to sluice away any remaining loose rock from the top of the rock fall site.
"This rock fall is larger than one two years ago about 100 metres further north."
The speed of the clearing work would depend on the size of the rocks, Mr Jarvis said. There were indications large slabs of rock were buried in the debris.
"We understand our customers will be inconvenienced by this situation and are making every effort to get at least one lane of the highway open as soon as it is safe to do so," Mr Jarvis said.
There was enough equipment and personnel on-site to begin removing debris as soon as it was safe to do so, he said.
Recent heavy rain was believed to have caused the rock fall.
There would be a further update on the highway closure at midday tomorrow.
The road also remained closed further west on the highway, at the Diana Falls slip site near Haast Pass, until at least tomorrow morning.