Emergency services were called to the crash on the Twizel-Omarama Rd (State Highway 8) about 12.30pm. No other vehicles were involved.
Senior Sergeant Jason McCoy, of Oamaru, confirmed that a man was killed. St John ambulance said another person suffered moderate injuries.
Snr Sgt McCoy said inquiries were continuing and witnesses were being spoken to.
Any witnesses who had not made themselves known to police were asked to contact the Oamaru Police Station.
State Highway 8 is down to one lane under traffic management, 2km south of the intersection with Lake Ohau Rd.
Police asked people to continue to avoid the area and consider delaying travel.