Lawyers for the United States peace activist Scott Parkin are in court to defend his right to know why he was deported from Australia.
Mr Parkin was ejected last year after the Australian Security Intelligence Organisation made an adverse security assessment of him.
The contents of the assessment were never disclosed to Mr Parkin.
His lawyer, Anne Gooley, says she is optimistic the federal court will protect her client's civil liberties.
"This is an important case, not just for Scott.
"It's an important case for the remaining two refugees on Nauru, who are in identical circumstances to Scott, which is they get an adverse security assessment and then they're told, 'You can't challenge it'."