The Federal Government has dropped people smuggling charges against an Indonesian boy after his lawyers travelled to Indonesia to prove his age.

Dion Domun was intercepted off Ashmore Reef in March this year, and charged with aggravated people smuggling.

He was moved to immigration facilities in Melbourne.

Mr Domun always maintained he was 15, and Legal Aid lawyers travelled to his village in Indonesia to get affidavits from his family to prove he was under 18.

Based on that evidence the prosecution dropped the charge.

Mr Domun will now be released and returned to Indonesia.

The boy's lawyer, Gavin Green, says his client is thrilled.

"He knew this was going to happen," he said.

"He was very excited yesterday and we're delighted for him."

Under Federal Government policy only adult crew members on boats bringing asylum seekers face charges, while minors are sent home to their families.

But it can often be hard to prove the ages of Indonesian crew members and activists say up to 100 people who say they are children are in jails around Australia awaiting trial under people smuggling laws.

Federal police use a wrist X-ray technique to establish the age of people in detention, but the technique is widely questioned by doctors and the courts.

Earlier this week, a human rights barrister called for a judicial inquiry into the plight of young Indonesians in adult prisons in Australia.

Barrister Mark Plunkett, who has been enlisted by the Indonesian government to help identify cases, launched a blistering attack on the Federal Government, accusing it of being party to institutionalised child abuse.

"The Commonwealth is cruel to children. This is institutionalised child abuse by the law enforcement authorities who won't do their basic duties and find the mums and dads," he said.