The controversy over the role of a black surgeon in the world's first successful heart transplant has been reignited by the release of a documentary film in South Africa.

Hidden Heart tells the story of Hamilton Naki, a gardener turned self-taught surgeon who became a key assistant to pioneering heart surgeon Dr Christiaan Barnard in his organ transplant programme.

While Barnard was feted around the world for carrying out the first heart transplant in 1967, Naki was effectively rendered invisible by the apartheid government, retiring in 1991 on a gardener's pension equivalent to £70 a month.

But a decade later, with Barnard's help, his contribution gained international recognition. "Hamilton Naki had better technical skills than I did," Barnard said. "He was a better craftsman than me, especially when it came to stitching, and had very good hands in the theatre."

Hidden Heart shows how the operation was trumpeted as a public relations coup for the Afrikaner people and the apartheid government. One review of the film notes that one of the proudest moments in South African history "might also have been one of our most shameful".

Naki, born to a peasant family, had no formal education and started work as a gardener at Cape Town University at 14. One day, while rolling the tennis courts, he was invited by Barnard's predecessor to step inside the laboratory and help with an experiment on a giraffe. He worked his way up to be principal laboratory technician and taught some 3,000 surgeons. Under apartheid laws, Naki officially remained listed as a gardener and cleaner. He was finally recognised with an honorary master of medicine degree in 2003.

The Swiss-made documentary reopens the debate over Naki's claim that he participated in the historic first heart transplant. After Naki's death, aged 78 in 2005, the Economist published an obituary saying Naki assisted in the transplant, but later published an amendment saying it had been assured by surgeons at Groote Schuur hospital that Naki was nowhere near the operating theatre at the time.

Cristina Karrer, co-director of Hidden Heart, told the Star newspaper the mystery ultimately matters little. "To me the point is that he was there in the process," she added. "His contribution was enormous, taking into consideration where he came from."