Bennett was born in Amersham, Buckinghamshire to Louis Bennett and Lily Bennett (née Black). He was educated at the Hall School in Hampstead, then moved with his family to Australia in 1951 and attended the Scots College, Sydney. He subsequently attained degrees in Arts and Law (with honours) from the University of Sydney and an LLM and SJD from Harvard Law School. His father, Louis Bennett, was a distinguished solicitor in Sydney, and as a Provost Major was in charge of the escort that took the Nazi "Oberkommando der Wehrmacht" to Nuremberg for trial. Bennet's Aunt Major Jane Bennett  served at the GCCS at Bletchley Park where the British broke several Nazi codes.