A growing number of people smugglers who claim to be under 18 to avoid being jailed in Australia have turned out to be adults.
Federal police officers have carried out 100 wrist X-rays of Indonesian crew members claiming to be under the age of 18 since 2008.
But results of the scans have shown 60 of the so-called "children" to be aged over 18.
Federal Government policy is that children found working as crew members on illegal boats are not charged and are deported instead.
Adult crew members face a mandatory five-year jail term.
Federal police officers can order a wrist scan, examining the person's skeletal development, if they believe a suspect may be lying about their age.
The Department of Immigration and Citizenship uses a focused interviewing technique to ascertain age and can also take AFP wrist scans into account.
In a recent department trial of 60 immigration detainees, the focused interviewing technique found that half the detainees claiming to be children were in fact adults.
"The issue of age determination is complex, with many clients who arrive as asylum seekers or crew on irregular maritime ventures presenting with no proof of identity and often not being able to provide a date or even a year of birth," the spokeswoman said.
"Other ways in which the department tests and verifies claims, in relation to age, may be by contacting family members through contact details provided by clients.
"When these checks are undertaken, departmental officers identify themselves and explain the reasons for the calls. Interpreters may assist in this process."
Of the 1034 children currently in immigration detention, 40 per cent of them arrived in Australia without any accompanying family members.
The revelations come as one male teenager, who claims to be 16, fights people smuggling charges through the courts after his wrist scan determined he was 19.
A Perth magistrate is expected to hear evidence on his age later in the year.