Stateless refugee Fazel Chegeni Nejad survived torture in Iran only to later die on Christmas Island.
An inquest into his death in November 2015 will begin on Monday in the West Australian Coroner's Court.
The National Justice Project, which is a not-for-profit legal service representing Mr Nejad's family, says he was denied appropriate mental health care.
Lawyer George Newhouse said Mr Nejad told doctors in 2012 that unrest on Christmas Island depressed his mood, triggered his anxiety and caused mental health problems.
"Fazel became mentally ill as a result of his prolonged incarceration without proper diagnostic care," Mr Newhouse said in a statement on Sunday.
"Our government knows that victims of torture should not be locked up like animals, but they ignored the warnings and poor Fazel's desperate pleas for mental health support."
Mr Newhouse said Mr Nejad was a quiet and gentle man, who was loved by his family and fellow refugees.
"Fazel's family want to know who will be held accountable for his death."
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