Police Inspector Wayne Ewers said the tourist died instantly at the Te Paki sand dunes on Monday afternoon.
The accident happened at about 1.40pm as the man sandboarded down Te Paki's famous dune to the stream.
It appeared neither he nor the bus driver saw each other.
The tragedy was witnessed by a large number of people, including the victim's wife, son, daughter-in-law and granddaughter, who had been planning to return to South Korea later this week.
The bus driver, employed by a Kaitaia company, was badly shaken, Ewers said, but neither he nor anyone else had been injured.
The bus company sent another vehicle to Te Paki to collect the passengers.
The police Serious Crash and Commercial Vehicle Inspection units were both investigating, while WorkSafe NZ was also likely to conduct an inquiry. The bus had been impounded for an examination.
The man's body had been taken to Kaitaia, and the death had been referred to the coroner, who would decide whether a post-mortem examination would be required.
