Witness Jay Cameron (22) said he saw the man run and stand in front of a large Coca-Cola truck on the one-way section of Gt King St, in Dunedin, shortly before noon.
The driver swerved his truck to avoid the man, who was described as being in an agitated state, before the pedestrian returned to the side of the road, he said.
Moments later, the man ran into the path of a smaller contractor's truck which struck and propelled him to almost the other side of the 50kmh area of the state highway.
Mr Cameron said he went to assist the man along with the truck driver, who told him: "I think I just killed him".
"He was groaning and covered in blood."
The man was taken to Dunedin Hospital by St John with injuries which were serious but "not life-threatening", Senior Sergeant Steve Aitken said.
The man, who was aged in his 20s, was yet to speak to police about the incident.
"It appears to be a deliberate attempt by this male to get himself run over," Snr Sgt Aitken said.
