A Country Fire Service (CFS) volunteer responding to a grass fire has been killed after being hit by a fire truck in South Australia's south-east.

The incident happened about 4:30pm on the Southern Ports Highway at Rendelsham, 12km north-west of Millicent.

Police said the man was standing behind his CFS ute on the highway preparing to fight the fire when the CFS fire truck hit him.

The man died at the scene, police said.

The Southern Ports Highway between Millicent and Rendelsham was closed in both directions and traffic diversions were put in place.

Major Crash investigators were sent to the scene.