The accused, who cannot be named for legal reasons, pleaded not guilty to a single murder charge in the NSW Supreme Court in Sydney on Monday.

Crown prosecutor Lee Carr told the trial new information had come to light that morning which indicated a special verdict may be made under new laws centred around mental health and cognitive impairment.

He handed up the Crown bundle of evidence, which included body-worn footage from police at the time of the arrest and facts both the defence and prosecution agreed on.

Details of the case, including the identity of the deceased and the nature of her fatal injuries, have been suppressed by the court after a young girl was found dead on the morning of July 8 last year.

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Public defender Stuart Bouveng argued the teenager may not be held criminally responsible for the alleged murder if the evidence shows she was negatively affected by a mental health condition or impairment at the time.

He made the case that expert medical reports handed up to the court made it "clear there's a mental health impairment".

The court heard there may be no need to call the experts to give evidence.

Acting Justice Carolyn Simpson adjourned the case to consider the legalities of whether the new act or older legislation would apply to the teenager's case.

"I see little benefit in taking up time with having oral evidence repeating what's already been said in writing," she said.

She explained to the teenager that some expert evidence may be called in the judge-alone trial.

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

The case will return to the NSW Supreme Court on Tuesday.

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