An investigation into abuse at an Adelaide youth detention centre has led to the arrest of a 73-year-old man, who will face court accused of dozens of sexual offences — many of them against children.

The man, from Adelaide's western suburbs, is accused of crimes against 29 survivor-victims over more than two decades, and has been charged with 55 offences.

Police said the man had access to the now-closed Magill Training Centre but was not a government employee.

Charges against him including multiple counts of rape, unlawful sexual intercourse, sexual abuse of a child, gross indecency and indecent assault, police said.

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