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 Monday.

Supported by his grandmother and sister, he pleaded guilty to two counts of culpable driving causing death. He also admitted a charge of driving as an unaccompanied learner.

Each of the charges related to a single-vehicle crash that occurred on the Monaro Highway on October 9, when the boy was behind the wheel of a red Toyota Camry.

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 when emergency services discovered the girls' bodies in the car, having been notified of the crash by a passerby.

Police subsequently identified two male teenagers who had left the scene of the incident.

They included the boy who has now admitted being the driver.

The 16-year-old, who was on bail at the time in question, is also charged with failing to stop and render assistance after the crash.

His lawyer told the court on Monday that she was waiting on the results of an autopsy report, which should be available in four weeks, before entering a plea to that charge.

The boy, who remains behind bars on remand, is due back in court on December 12.

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Agreed facts outlining the boy's crimes are yet to be tendered to the court.

However, in the immediate aftermath of the crash, Detective Acting Inspector Brian Tadic said the Camry had "sustained extensive damage, to the point where it is unrecognisable".

He also said speed and wet conditions may have been factors in the crash.

Another senior ACT Policing officer, Acting Inspector Travis Mills, addressed media the day after the fatal crash, when the victims had been due to start the current school term.

He said teenagers should not get into a car if they thought it was unsafe.

"I would urge passengers to ask before jumping into a vehicle," Acting Inspector Mills said.

"These young girls were going to return to school today.

"Our kids are smart. They need to start thinking and behaving in such a way on our roads so they arrive home safely."

The ACT Education Directorate has previously said public schools were offering extra support to students and families affected by news of the double-fatality.

The girls' deaths were the 13th and 14th to occur on ACT roads this year. The toll now stands at 18, which is the worst figure in the territory since 2010.

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