SYDNEY - A 13-year-old who believes she is a girl born into a boy's body has won permission to undergo the first stage of gender-changing treatment, with at least four other NSW adolescents hoping to secure similar rights.

The 13-year-old was granted permission to undergo controversial puberty-blocking treatment by the NSW Family Court three weeks ago, in an application supported by family members.

The child's psychiatrist, Dr Louise Newman, said they sought the court's permission to use the hormones to delay pubertal changes because they would prove distressing and unacceptable to the child.

Lawyer Rachael Wallbank, herself a transsexual, said she presented the Family Court with evidence that puberty-blockers were reversible and could save the child from self-harm or suicide.

"All the medical evidence, all the lived experience of people with transsexualism like myself however, and all of the post-treatment studies of children indicate that the earlier the children with transsexualism receive this treatment, the better their lives are, the happier they are, the more they can actually live out a useful and fulfilling life," she told ABC's Four Corners in a program due to screen tomorrow night.

Ms Wallbank will soon return to court seeking a precedent-setting order that could remove the requirement for children to get Family Court approval for sex-change treatment.

This could clear the way for the four other teens to begin treatment without a legal battle.

"In my view, once a court hears more complete evidence about transsexualism ... then the court will be more comfortable about allowing the treatment of transsexualism in childhood to follow ... a medical course rather than imposing on parents of these children the additional financial and mental burden of having to take the child through a legal process to enable that child to receive the treatment it needs," she said.

The NSW court's recent decision follows the Family Court ruling last year that will allow Melbourne 13-year-old Alex, who was born a girl but identifies as a boy, to change gender, beginning with testosterone treatment at age 16.

Professor Garry Warne, of Melbourne's Royal Children's Hospital, believes a compromise has to be reached on such treatments whereby the courts agree to certain guidelines.

He believes puberty blockers shouldn't be given to children aged under 16 years, despite advocates saying they can be used from age 12 or 13.

"They're worried about making the wrong decision and they're worried about it coming back to bite them later on," Prof Warne said.

The NSW children's bids to change sex are revealed as part of the Four Corners report on new research into what determines gender, with science now suggesting gender is dictated not just by chromosomes but by "brain sex" -- a hard-wiring of the brain before birth.

As many as 40,000 Australians don't have standard sex chromosomes.

- AAP