The boy was arrested in February this year and charged with people smuggling, after being picked up on an asylum seeker boat on Ashmore Reef in April last year.
The convener of the Human Rights Alliance, Gerry Georgatos, says the Australian Federal Police classified the boy as an adult with the help of a discredited bone X-ray technique.
Mr Georgatos says he has spoken to the boy's relatives in Indonesia and they say he is only 16.
"I've gone to parliamentarians, I've gone to every relevant authority, there are no excuses," he said.
"If this was Australian children in prisons in Indonesia it would be an international incident and also we've tainted these people as people smugglers when all they actually are, are kids who have worked as deckhands or cooks on a boat."
The opposition Labor party in Western Australia is calling on the State Government to free the boy.
Labor's spokesman for Multicultural Interests, John Hyde, said up to 20 Indonesian children were currently detained in Western Australian jails.
He says the minister, Terry Redman, has known about the issue for over five months.
"It is unbelievable that the minister for prisons is not taking his role seriously," Mr Hyde said.
"He's more concerned with talking about cows in Indonesia than Indonesian children who are locked away in our adult prison."