Emergency services were called to the accident on the Katiki railway overbridge, between Shag Point and Moeraki, at 12.15pm.
The southbound van appeared to have crossed the centre line and struck the northbound stock truck and trailer unit, Acting Sergeant Craig Sew Hoy, of Oamaru, said.
A passing doctor was one of the first to help, he said.
A male tourist, believed to be in his late 20s, died at the scene.
Police were yesterday working to identify the man and contact his next of kin, with more details expected to be released today.
The truck driver and the stock on his truck were unhurt.
The truck was towed from the scene.
State Highway 1 was closed and traffic diverted through Trotters Gorge, while heavy vehicles waited for both lanes to be reopened at 5pm.
