The 16-year-old girl, who cannot be named, admitted she threatened a taxi driver with a knife before stealing his car on February 27, two hours after the legislation came into effect.
Under the new laws, children aged 14 and over who commit home invasions or aggravated carjackings will face the county court where penalties are tougher than a children's court.
But the girl's lawyer Kate Perry on Friday argued the teen should still be sentenced to probation without a conviction because her rehabilitation was the priority.
"She never wants to be involved in serious offending again," Ms Perry told the Victorian County Court.
The balaclava-clad girl and her male co-offender jumped into the back of the victim's taxi about 2.47am on February 27, the court has been told.
After the driver told them to leave, the girl pointed a knife at his neck and demanded he get out of the car and leave his phone and keys behind.
Fearing he would be stabbed, the driver stepped out and called for help as the girl drove the car away from Dandenong train station in Melbourne's southeast.
The boy was in the front passenger seat.
The pair crashed a short time later after speeding along the Princes Highway at 180km/h.
The boy was in March sentenced in a children's court to a 12-month good behaviour bond after pleading guilty to carjacking.
But the girl is facing the higher court after pleading guilty to the more serious aggravated carjacking charge.
Ms Perry argued her client should be spared prison time because she was young, had no prior convictions and came from a depraved upbringing.
The teen also had diagnosed post-traumatic stress disorder, ADHD and possibly autism - all of which made her more impulsive, Ms Perry said.
The girl told a psychologist she had been intoxicated and her friends gave her the knife and instructed her to steal the car because she was the only one without a criminal history.
Prosecutor Steve Tamburro cast doubt over the teen's account, noting her male co-offender also had no prior convictions.
Mr Tamburro argued a term of imprisonment was the only appropriate sentence, noting the girl had shown limited remorse and there was a risk of her reoffending.
The prosecutor also said aggravated carjacking was an inherently serious offence, which was reflected in the legislative changes.
Judge Scott Johns accepted the seriousness of the crime but said he also needed to consider the girl's personal circumstances.
She will be sentenced in the Victorian County Court on Wednesday.
13°C