Immigration Department documents released under Freedom of Information laws detail how and why children as young as nine have harmed themselves in detention.

The incident reports detail 26 cases of children who harmed themselves between August 2010 and November 2011 in the Darwin Airport Lodge and Botanic Apartments.

They include a nine-year-old who tried to overdose on painkillers and stated he was "going crazy in detention".

There was an instance when a 10-year-old boy who cut his forearm and a 17-year-old who tried to hang himself while he knew guards were responding to another incident.

The Darwin Asylum Seeker Support Network says the incidents show such places of detention are inappropriate for children.

Professor Louise Newman, who chairs a Federal Government panel advising the Immigration Department, says she has studied the reports.

"Fundamentally what it says to those of us in mental health and health professions is that children should not be detained," she said.

Fernanda Dahlstrom from the Darwin Asylum Seeker Support Network says the cases are horrific.

"The one that really stands out is a nine-year-old performed an act of self harm involving taking an overdose of pain killers," she said.

The Immigration Department says self-harming has decreased significantly since 2011.

While it cannot supply numbers, it says the reduction is due to improved mental health services and training for staff, less time spent in detention and the use of bridging visas and community detention.