A 14-year-old girl and a 12-year-old boy were in a stable condition at Melbourne's Royal Children's Hospital on Wednesday night, after the Holden Captiva they were travelling in collided with a truck on the Hume Freeway at Locksley about 8.20am.
The 15-year-old girl in the front passenger seat of the car died instantly.
The 14-year-old girl allegedly behind the wheel was under police guard in hospital.
The Holden was stolen near Wangaratta in Victoria's northeast on Tuesday night and was spotted driving erratically on the morning of the crash.
Officers believe the three were friends from the Wangaratta area.
Victoria Police eastern region Superintendent Michael Layton said there would be consequences for the driver.
"This is a tragic scene and high risk-taking behaviour by youths," Supt Layton told reporters on Wednesday.
"It's totally avoidable ... (and) it's probably every parent's worst nightmare who might be dealing with rebellious teens and young people."
Police were on Wednesday yet to determine the exact circumstances of the crash, with Supt Layton saying the truck was in the left-hand lane of the Hume Freeway.
Both the truck and the car were travelling in the same direction when the incident happened.
The 45-year-old truck driver, Clint Sneddon, was uninjured.
He said he was driving at 99km/h when the car hit his truck hard and he was jolted.
"I don't know what it is with these young kids, I don't understand what they get out of that," Mr Sneddon told Nine News.
"Look where it has got them. One is dead and (the other two) have got to suffer for the rest of their life and deal with it."
Police have urged anyone with information about the crash to contact them.
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