A truck carrying steel has rolled in the Wairau Valley near Blenheim, spilling diesel onto the road.
The truck was on its side and was blocking one lane of State Highway 63, just west of Wash Bridge, a police spokeswoman said.
The driver wasn't hurt and no one else was involved.
The road is closed and won't reopen until 6.30pm at the earliest.
The incident happened about 3pm but council contractors were still at the scene cleaning up the diesel spill about 5pm, a police spokesman said.
