The Federal Government is standing by the use of a controversial method to determine the age of suspected people smugglers, despite its repeated failure.

The Government relies heavily on wrist X-rays to determine whether a person can be prosecuted as an adult or if they should be sent home because they are under 18.

Lawyers and doctors say the technique is unreliable, and yesterday a 34th people smuggling case collapsed after the defence proved the X-ray was inaccurate and the accused person was just 15.

Minister for Home Affairs Brendan O'Connor says the Government does not want to prosecute children, but says it will continue to use the age determining method.

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Mr O'Connor added the practice had long been used to determine the age of criminal suspects.

"The AFP conducts an age determination process in accordance with the Crimes Act which includes a number of measures including X-rays of wrists," he said.

"And indeed that has been one of the ways in which age has been determined for a very long period for all crimes. But it's not the only matter that's considered.

"In the end, of course, the AFP does not decide whether the person's a minor or not. They assert it and they assert the person either is or isn't, and they provide evidence to substantiate their assertion."

Mr O'Connor denied the controversial X-ray method was the key mechanism that is used.

"The other way is documentary evidence," he said.

"What we do is seek out further information. We've changed the arrangements in terms of improving criminal justice measures.

We offer dental X-rays as well to alleged people smuggling crews. We've taken steps as early as possible to seek information from individual country of origin, including birth certificates.

"We seek these matters out and indeed we also use interview techniques to help determine age. There's a variety of ways we seek to determine the veracity of assertions as to whether a person is a minor or not."

Mr O'Connor confirmed the X-ray method is not always reliable.

"There's been a few opinions as to its reliability. So I understand it is a very strong indicator about a person's age," he said.

"But that's why we don't rely upon it entirely. It's not 100 per cent. It is something that gives very strong indication and that's why we rely upon supplementary evidence to consider that matter.

"It will continue to be used along with other evidence. It's something that corroborates other evidence and that's entirely proper. People have different views on that but we'll continue to use it," he said.