A serious crash has closed a section of a major Southland highway, as significant traffic congestion starts to build.

Police said State Highway 6/Great North Rd was closed near Eglinton St in Winton, about 30km north of Invercargill.

The two-vehicle crash was first reported to police just after 3pm.

Police said initial indications suggested the people involved had suffered serious injuries.

Motorists are advised to avoid the area and expect delays.

Police said detours were in place.