A falling boulder has killed a man in a car near Murchison, closing State Highway 6 tonight and all day tomorrow.

Police are investigating the accident at O'Sullivan's Bridge just before 2pm. Photo: Google Maps

Police are investigating the accident, which happened at O'Sullivan's Bridge just before 2pm.

Police said a 59-year-old Canterbury man died after the incident.

NZTA system manager Andrew James said there was no simple detour in the area - a main route between Nelson and Christchurch, so drivers may need to delay their journeys.

Travellers between Nelson and Christchurch are advised to take State Highway 1 through Kaikoura.

The detour for drivers between Nelson and the West Coast is through the Lewis Pass.