Cameron Forsyth, 38, had pleaded guilty in the Downing Centre District Court to aggravated dangerous driving occasioning death under the influence of drugs.

He crashed into and killed 17-year-old Adam Ghoz, who was walking to soccer practice along the Hume Highway in Casula on January 7, 2021.

Judge Ian McClintock on Friday jailed Forsyth for seven years with a non-parole period of four years.

He also referred Forsyth to the compulsory drug treatment program at Parklea jail.

In her victim impact statement, the teenager's mother Rima Ghoz spoke of the moment her life was turned upside down.

She had touched down in Lebanon to visit her sick sister, messaging all her children.

They all replied except for Adam.

While he was dying in hospital from critical injuries, Ms Ghoz could not return home in time due to COVID-19 quarantine restrictions.

Now she visits his grave every day, no matter the weather, as she deals with the "highest level of grief a person could experience", losing a child.

"Adam was loved by everyone", so special and kind-hearted he invited a bullied child into his school friendship circle, without him knowing, she said.

The family used to gather for every meal but now hide away from each other in pain, and no longer feel safe after Adam died on a clear day, just walking on a footpath, she said.

Forsyth gave evidence at his sentence hearing, describing himself as a high-functioning drug addict since he was a teenager.

He had been on and off heroin-replacement substance therapies before the fatal crash.

Four months earlier he had nodded off and run into the back of a car but was never charged and returned to opioid replacement therapy, he said.

But a phone argument on January 7 with his partner triggered traumatic childhood memories and he went to his dealer's house for a "big shot" of heroin.

The next thing he remembers is waking up after his car had mounted a kerb, a man telling him "I think you've hurt someone," and seeing a teenager being administered first aid.

He will never forget this vision, he told the family in a letter to the court.

"There is no way to justify the charge I stand here for ... I cannot begin to imagine the pain and grief I have caused."

He apologised to Adam's friend who was walking beside him that day, to the first responders, to his friends and teachers, and to the wider community.

"I've ruined my family, I've ruined another family.

"The only thing I can do in Adam's memory is to better myself as a human.

"I hope in time they can find inner peace or find comfort in my feelings of guilt and burden (that) is something I will carry for the rest of my life."

Australian Associated Press