The teenager accused of murdering a 10-year-old girl she knew at a country NSW property in Gunnedah was mentally unwell at the time, her trial was told.

Now 15, she pleaded not guilty in the NSW Supreme Court on Monday to murdering the younger child on July 8, 2020.

Both children’s names and the fatal injuries suffered have been suppressed by the court.

Acting Justice Carolyn Simpson was told two expert reports found the accused was acting under a “clear mental health impairment” that was undisputed.

Prosecutor Lee Carr SC said the proceedings should be dealt with under the new Mental Health Act that allows for a special verdict.

The teenager may not be held criminally responsible if the evidence establishes she was negatively affected by a mental health condition or cognitive impairment at the time.

Mr Carr tendered a crown bundle of evidence including body-worn footage from police at the time of the arrest and facts.

Justice Simpson explained to the teenager that some expert evidence may be called before the judge-alone trial on Tuesday.

“Otherwise proceedings will conclude until I deliver a verdict,” she said.

– AAP