One person has been critically injured in a two-vehicle crash on State Highway 1 at Riverlands, south of Blenheim.

The crash, at the intersection of Sheffield Street, was reported to emergency services around 4.20pm on Monday.

A police spokesperson said the highway was blocked and ambulance services were at the scene.

The Serious Crash Unit has been advised.

Motorists are urged to avoid the area and take alternative routes or delay travel, with the highway expected to be closed for several hours while emergency services work at the scene.