MELBOURNE - A court has permitted a teenage girl to have her breasts removed so that she can better resemble a boy.

The 17-year-old, codenamed Alex, has been on hormone treatment ordered by the Family Court to prevent menstruation and breast development since she was 13, The Age newspaper said.

She returned to the court in December last year to seek permission for a double mastectomy so she would look more like a boy, the newspaper said.

The Chief Justice of the Family Court, Diana Bryant, said it was in the teen's best interests to have the surgery immediately rather than wait until she turned 18.

Alex has been diagnosed with "gender identity dysphoria", in which a person has the normal physical characteristics of one sex but longs to be the opposite.

"In the end, it wasn't a particularly difficult issue because the only real issue was, 'Would he (Alex) have it at 17 or once he's 18?' Then, he doesn't need permission," Justice Bryant told The Age.

"So the issue was: 'Was there any likelihood he would change his mind in the meantime, and was it in his best interests to have it at that time?'

"Overwhelmingly, the evidence was that it was in his interests. And I made that order. I wanted to make it quickly so that he could have the operation straightaway."

But ethicist Nick Tonti-Filippini said mainstream medicine did not recognise hormone treatments and surgery as treatment for gender dysphoria, and that under US guidelines it was psychosis because "it's a belief out of accordance with reality".

"What you are trying to do is make a biological reality correspond to that false belief," he told The Age.

Justice Bryant's written judgement will be published soon.

Justice Bryant said it was better for the teenager to have the surgery at 17 as she would qualify for support from state social services, and it was also a crucial time in her social and mental development.

- AAP