The legal requirement for transgender children to receive court approval before they can begin hormone treatment is being challenged by a 17-year-old in a watershed case.

The teenager, known for legal reasons as Kelvin, was born female but began to publicly identify as a male at age 13.

Privately he identified with the concept of transgender at age nine after reading about it in a book.

He was eventually diagnosed as having gender dysphoria but did not undergo treatment in time to prevent female puberty.

An earlier hearing in February heard Kelvin took the oral contraceptive pill to stop menstruation but experiencing puberty caused him "significant distress".

In 2013, he cut his hair short, began dressing as a male and purchased a chest binder.

He changed schools in Year 8 and presented himself as a male from day one.

His doctors recommended he commence stage-two treatment, which involved the administration of the male sex hormone testosterone.

The hormone induced facial hair, a deeper voice, increased muscle and the cessation of his periods.

Under current law set by the Family Court, stage-two treatment is classified as a "special medical procedure".

This means parents of children under 18 cannot approve treatment in the same way as they would in the ordinary course of their authority as parents.

'It's not surgical, it's not medical'

At the February hearing, a judge found Kelvin fully understood what stage-two treatment would mean, and gave approval for the treatment to commence.

But Kelvin's father, and various medical, legal and transgender groups, believe the requirement for court approval should be abolished in some cases.

Proponents of change argue "time is critical" for successful treatment, and the court process can mean substantial delay and significant legal costs.

Today, the Full Court of the Family Court began hearing an appeal brought by Kelvin, with the support of his father.

About 100 people, including members of the transgender community, attended the hearing in Sydney.

The court made an order to allow Kelvin himself to sit in the court.

Counsel for Kelvin's father, who, as with all the lawyers appearing in the case cannot be identified for legal reasons, told the five judges sitting on the appeal: "A recalibration of the court's supervisory power is warranted."

She said while stage-two treatment was "a pretty powerful thing, it's not surgical, it's not medical, it's not invasive".

"The nature of the treatment in stage two simply doesn't attract that level of supervision."

The court can only authorise treatment after a paediatrician, at least one psychiatrist and a second mental health professional and a fertility expert agree treatment is in the child's best interests.

Court proceedings 'costing families thousands'

Some of the characteristics of cross-sex hormones are irreversible, such as voice deepening.

In the case of a young person who takes female hormones and develops breasts, surgery may be required to reverse the changes.

The next level of treatment — gender reassignment surgery — is irreversible and is only performed on people over the age of 18, when they are no longer under the jurisdiction of the court.

The court heard that between July 31, 2013 and August 16, 2017 the Family Court dealt with 63 cases involving applications for either stage-two or stage-three treatment.

All but one were approved.

However in its submission, the NSW Department of Family and Community Services said the fact that many of the cases had been determined without controversy, "is not an argument in favour of dispensing with the requirement of scrutiny".

The Royal Children's Hospital gender services has received 126 patient referrals this year and 710 since 2003. Of these, 96 per cent diagnosed with gender dysphoria continued to identify as transgender into late adolescence.

No patient who commenced stage two treatment has sought to transition back to their birth sex.

But FACS submitted the persistence of childhood dysphoria into adulthood is still the subject of studies and further evidence is required.

A study of 12 families by Dr Fiona Kelly of La Trobe University found the average cost of court proceedings was between $8,000 and $30,000.

Dr Kelly found it took on average eight months from filing an application with the court to treatment commencing.

The Commonwealth Attorney-General, the NSW Department of Family and Community Services, A Gender Agenda, Australian Human Rights Commission and The Royal Children's Hospital, Melbourne are also parties to the hearing and will make submissions over the next two days.