A 20-year-old man has been charged with manslaughter over a crash on Dunedin's Southern Motorway in May.

Two people, including the driver's girlfriend, were killed in the smash near the suburb of Green Island.

The man charged with manslaughter, who is from Milton, is also facing dangerous driving charges.

He is due to appear in the Dunedin District Court on November 18.

The driver's girlfriend and her brother were killed.