The 16-year-old boy, who cannot be named because of his age, faced the ACT Children's Court via audio-visual link from the Bimberi Youth Justice Centre on Wednesday.

He pleaded guilty to a charge of failing to render assistance after the crash.

Members of the dead girls' families were present in court, watching tearfully from the public gallery.

The boy previously pleaded guilty to two counts of culpable driving causing death and driving as an unaccompanied learner.

The charges relate to a single-vehicle crash on the Monaro Highway on October 9 last year.

The boy, who was on bail at the time, has admitted to being behind the wheel of the red Toyota Camry when it crashed.

ACT Policing previously said that at an unknown time that morning, the car left the roadway, crashed through a row of temporary fencing and hit a tree, killing passengers Susi Kopysiewicz, 14, and Claire Sankey, 15.

Susi was a day away from celebrating her 15th birthday.

Emergency services discovered the girls' bodies in the car, having been notified of the crash by a passerby at around 8am that day.

The girls are believed to have died some time before police and ambulance crews arrived at the scene.

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Later, police identified two male teenagers, including the 16 year old boy, who left the scene of the incident.

Agreed facts outlining the crimes have not been tendered to the court.

The boy is set to be sentenced in the ACT Supreme Court. The case is scheduled for mention on February 2.

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