Traffic is building for kilometres down State Highway 1 in southern Te Tauihu o te Waka/Marlborough following a serious crash on Thursday.
Police have since closed the highway between Ward and Kekerengū, after the two-vehicle crash was reported just after 11am.
No local detours are available.
The only alternative road is SH7, via Lewis Pass, and SH63 via Wairau Valley. These roads have significantly longer travel times.