A police spokesperson said the single-vehicle crash happened on Tekapo-Fairlie Rd (SH8) at about 3.17pm.
One person died at the scene and another sustained moderate injuries, the spokesperson said.
An Australian woman visiting New Zealand, who declined to be named, said she was one of the first on the scene of the crash, and saw two men in a work van trapped in the vehicle.
"We don’t know how many times the vehicle spun, but the van was on its roof with the driver and passenger stuck inside."
The woman said she and another tourist helped direct traffic away from the crash while more people moved debris off the road and provided what medical assistance they could to the people trapped.
She said people went up to the van to attempt to try get the two occupants out of the van, but were unable to do so as both of the doors were stuck.
She stayed and continued to direct traffic until Fire and Emergency and Hato Hone St John arrived at the scene.
"Everyone present was just being amazing in responding, either to remove potentially dangerous goods from vans or clear the road — nobody got in anybody’s way.
"A local farmer cut through his fence to enable people to access the two men trapped in the van, everybody was just trying so hard to help.
"It was just awful, we cannot imagine what the families are going through."
State Highway 8 remains closed.
Detours are in place and the serious crash unit has been advised, the police spokesman said.
