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 confirmed two people died in the crash, while two others were taken to hospital with moderate injuries.
Emergency services have closed SH1 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.
Motorists planning on driving between Kaikōura and Waiharakeke/Blenheim have been urged to delay their travels and avoid the area - or expect lengthy delays.
Ferry users should factor the road closure into their plans.
Meanwhile, emergency services have rushed to another crash on SH6, the Haast Highway, in Tasman.
Police were notified about the two-vehicle crash at about 2.30pm.
"Initial indications suggest there are injuries," a police spokesperson said.
SH6 has been blocked and motorists are urged to avoid the area.
