A former Christian Brother has pleaded guilty to historical sexual assault charges dating back to his time working at a South Melbourne orphanage in the early 1970s.

At the County Court in Melbourne on Monday, Rex Francis Elmer, 75, pleaded guilty to two counts of unlawfully and indecently assaulting boys under the age of 16.

Rex Francis Elmer after an earlier court appearance. Credit:AAP/Daniel Pockett

At the time of the abuse, Elmer was working at St Vincent's Boys Home, a South Melbourne residential facility for wards of the state.

The 75-year-old admitted to assaulting a boy between January 1, 1971 and December 31, 1972.