About 100 underage Indonesian boys are languishing in adult prisons in Australia because the federal government is refusing to acknowledge their real ages, activists say.

The boys, some as young as 13, were mostly cooks or deckhands on boats that brought refugees to Australia, a University of New South Wales (UNSW) Law School public forum will hear on Monday.

They now face being convicted of people smuggling as adults after their age was determined by the use of controversial wrist x-rays, said Gerry Georgatos of the Human Rights Alliance.

"Most of these boys have never been in trouble before, all of them are impoverished," he told AAP on Monday.

"Their mental state is one of despair, one of fear of the unknown."