An intellectually disabled man kept in an alarmed unit for 25 years and only allowed in public while supervised is one of a handful of Victorians found to have been unlawfully detained by National Disability Insurance Scheme providers.

The 63-year-old, who has a long history of committing sexual assaults and other crimes, has lived under surveillance in a self-contained unit managed by disability services for decades without the legal orders and safeguards required to detain a person.