The 40-year-old driver suffered a broken jaw and cuts to his face and head in the accident. He underwent surgery last night.
The other passenger, a 39 year old man, has spinal injuries and was also having surgery last night.
The Greymouth Star understands the victims were bringing equipment to the new Sam Pan restaurant, in Albert Street. The restaurant, which was due to open on Friday, is owned by restaurateur Yu Ouyang.
Mr Ouyang has another restaurant in Dunedin, but of his two Christchurch businesses, one is in the process of being demolished and the other is stuck in the red zone.
He could not be reached for comment today.
Senior sergeant Allyson Ealam, of Greymouth police, said it appeared the driver had lost control of the rented truck while coming down the steep, narrow stretch of State highway 73 between the rock shelter and Windy Point.
The truck tipped on its side before crashing through the safety barrier and plunging down a sheer face to the riverbed below.
A preliminary check of the crash site found restaurant equipment, including food material believed to be cooking oil.
"At this stage we're sending a guy up to make an assessment on what needs to be done," West Coast Regional Council consents and compliance manager Colin Dall said.
The council was working with the Department of Conservation to co-ordinate the cleanup, because the food material would affect how they could dispose of the truck wreck.
Police have not yet been able to speak to the men about the circumstances leading up to the accident and the matter is still under investigation.