At least 33 people died after undergoing "deep sleep" therapy at a private Sydney mental hospital between 1963 and 1979, the opening hearing of an official inquiry was told.

Among the dead were nine patients at the former Chelmsford hospital who committed suicide, some long after the late Dr. Harry Bailey put them into a drug-induced coma for up to two weeks, lawyer Brian Donovan told the Royal Commission on Tuesday.A mother whose blind 14-year-old son underwent "deep sleep" treatment died four months after he was admitted to the hospital in 1969, he said.