A large boulder has blocked State Highway 6 on the South Island's West Coast.

Police say the rock was part of the cliff which runs along the highway near the West Coast town of Barrytown, it tumbled down the rock-face into the middle of the road.

The road has been closed due to the giant rock, and it won't be removed until midday on Sunday.

Police are recommending drivers take an alternate route if driving in the area to avoid a long delay.