A Queensland judge has given a pregnant 12-year-old girl, who has the intellect of a six year old, permission to undergo a late-term abortion.

In a landmark ruling, Queensland Supreme Court Justice Margaret Wilson ordered that doctors be allowed to terminate the girl's pregnancy because it was in her best interests.

No details were given about the circumstances under which the girl conceived, nor about the father, except that he is believed to live in Queensland.

Justice Wilson handed down her decision following an after-hours hearing closed to the public in Brisbane last week, but her finding was not released until Friday.

During the hearing, the court was told that the girl, who is 18 weeks' pregnant and in a public hospital, had sought to have her pregnancy terminated and had the support of her parents.

The matter was brought before court by state government lawyers, representing the girl's doctors.

The court heard that because of the girl's age and mental capacity, neither she, nor her parents, were legally able to give consent for a termination and a court order was required before it could go ahead.

It is the first case of its kind in Queensland.

The girl's obstetrician told the court the girl had the intellectual capacity of a six-year-old.

A psychiatrist who performed a mental status examination found her intellect was in the "very low normal range, possibly even lower".

Justice Wilson relied heavily on the opinion of two psychiatrists who told the court the pregnancy would pose serious dangers to her mental health and wellbeing, well beyond the normal dangers of pregnancy and childbirth.

They told the court the termination was the only way to avert potential dangers to the girl.

Justice Wilson ordered doctors to use the drug Misoprostol to induce labour because at this stage in the pregnancy there was a high risk that other methods could seriously harm the girl.

The court was told that her parents had been separated for three years but shared custody.

Her mother moved interstate about three weeks ago for work, leaving the girl with her grandmother and father.