A COURT has allowed a 10-year-old boy to become the youngest Australian to have sex-change therapy.

The boy, known as Jamie, has lived as a girl for two years, dressing in feminine clothes, using the girls' toilet at school and "presenting as a very attractive young girl with long, blonde hair", the court heard.

Jamie's parents, medicos and psychiatrists feared early-onset puberty could lead to self-harm or suicide and supported an urgent application for the child to receive sex-change therapy.

Family Court Justice Linda Dessau ordered that Jamie be allowed to start drug therapy to stop male puberty and that the court reconvene when she turned 16 to consider approving the second stage of taking female hormones to feminise the body.

The court heard Jamie saw herself as a "freak" and a "girl in a boy's body", and had first identified as a girl when she was a toddler.

Her mother said though doctors had told her Jamie would be the youngest patient to start such treatment, she was confident it was in her best interests.

She said before her child changed schools in year 3 and acted as a girl, she told her, "Mummy, it is so hard trying to be a boy", and that she had to "go to school disguised as a boy".

The mother said the family had started treating Jamie as a female in 2008 after her non-identical twin brother accepted her condition and announced: "I have a sister."

The mother was concerned that without treatment, Jamie's voice would break and an Adam's apple and facial hair would grow.

"(If Jamie does not receive treatment) she will very shortly develop obvious male characteristics ... which will be permanent and will not be able to be reversed," the mother said.

"At the moment Jamie can live comfortably as a girl, is socially confident and suffers no teasing or social isolation."

A medical expert said when he first saw Jamie in February 2009, she "looked convincingly female in every way" except her genitalia.

The doctor said he was concerned about Jamie's rapid-onset puberty, which was equivalent to that of a 14-year-old, and that sex-change treatment should start immediately.

Another doctor said stage one treatment to stop male puberty was reversible.