Bulgarian authorities should explain the legal basis for continuing to detain Australian Jock Palfreeman or release him immediately, Human Rights Watch says.
The 32-year-old was found guilty of murder in 2009 and sentenced to 20 years in jail, before being granted parole in September.
He is now being held in an immigration centre near Sofia, while he awaits the outcome of an unusual bid by Bulgaria's prosecutor-general to revoke his release.
"Palfreeman was ordered released by a high court in a final decision not subject to appeal. He has his passport and plane ticket. Australia wants him back. Why is he still detained?"Human Rights Watch researcher Lydia Gall said in a release.
"Under the rule of law, you can't just lock people up or prevent them from leaving just to make a point - there has to be a clear legal basis or it's unlawful."
Palfreeman has served more than 11 years behind bars over the fatal stabbing of Bulgarian Andrei Monov in a 2007 brawl.
He has maintained he acted in self defence after he came to the aid of two gypsies being assaulted by Mr Monov and his friends.
Palfreeman's lawyer, Kalin Angelov, has told Human Rights Watch that almost three weeks after his client was taken to Busmantsi immigration detention centre, neither of them has received an official document outlining the legal basis for his client's continued detention.