Lewis was an Oxford University don, a novelist, mediaevalist, literary critic, essayist, lay theologian and Christian apologist.
His seven fantasy novels for children, The Chronicles of Narnia, have sold millions of copies.
An article outlining Dr Tankard's discovery is to appear in the Times Literary Supplement, and a related academic article by him will soon appear in the Journal of Inkling Studies.
Dr Tankard was "quite thrilled" and "very gratified" his inquiries had proved successful after about ten years of part-time work, conducted entirely from Australia and New Zealand.
Long deeply interested in the works of Lewis, Dr Tankard said his curiosity had been aroused when he had first come across a reference to a television interview organised by British theatre critic and producer Kenneth Tynan, who had earlier been taught by Lewis at Oxford University.
Further inquiries showed Lewis had been interviewed and filmed for two episodes of the arts magazine programme Tempo, which had been produced for ABC Television (England) by Tynan in 1961-62.
The first programme, focusing on the erotic in literature, was never broadcast because of censorship restrictions.
Film versions of both programmes have been lost, but the interview transcript of the first show had been located, thanks to the help of many people, Dr Tankard said.
Mr Tynan's daughter, Roxana Tynan, who is now a human rights lawyer in California, had passed on a copy of notes from her late father's work diary, which shed light on the Tempo interviews.
Dr Tankard then learned the first interview had been conducted by a retired former English journalist, Wayland Young, the Lord Kennet.
He contacted Lord Kennet, who had written back, enclosing a copy of the missing interview.
• An Australian-born academic, Dr Tankard is a senior lecturer in the Otago English department. He joined the department in 2003.
