Nukyah Gunthorpe died in the early hours of December 17, 2018 when the car driven by her partner, Bailey Richard Hogan Jones, crashed into a truck on Somerville Street.

Jones, now 22, pleaded guilty in the County Court on Tuesday to culpable driving causing death, as he was affected by drugs to the extent that he did not have proper control of the vehicle.

The court heard Jones was travelling at high speed on Somerville Street when a truck approaching in the opposite direction moved into a turning lane, to turn right onto Keck Street.

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The truck driver moved to the left when he realised there was not enough time to make the turn, while Jones braked.

The car then skidded into the truck, colliding head-on.

Neither Ms Gunthorpe nor Jones were wearing seatbelts.

While paramedics performed CPR on Ms Gunthorpe until she arrived at hospital, she succumbed to her injuries a short time later.

Jones suffered serious injuries, while the occupants of the truck sustained minor injuries.

Police had begun to follow Jones shortly before the crash after seeing him driving at a fast speed, and at one point reached 130 km/h in the 60 km/h residential area in their attempts to catch up.

Jones said to a paramedic "I told her I didn't want to drive because I'd smoked ice", and told a police officer he had used the drug about two hours before.

Blood tests detected methamphetamine and amphetamine in his system, which a doctor said would have impaired his driving ability and would have prevented him from having proper control of the vehicle.

Ms Gunthorpe's mother, Louise Baggott, told the court the last thing her daughter said to her when she left the house before the crash was, "I love you, Mumma".

That night, she received a text message from Ms Gunthorpe saying she would be home soon.

She was now left with the images of her "beautiful" daughter's injured body in the hospital, Ms Baggott said, and she struggled to function.

"The impacts of Nukyah's death are so complex and so far-reaching," she said.

When the crash occurred, she said, the family was still mourning the death of Ms Gunthorpe and Jones' son Eli, who had died just a few months earlier at the age of nine days.

Ms Gunthorpe's younger sister told the court how her sister - "a really good cook" - always made her pancakes on her birthday.

But now she would no longer get to taste her food, and Ms Gunthorpe would not see her graduate or celebrate her 18th birthday with her.

She said she missed her older sister and found life difficult without her.

"The house is just so quiet now without her," she said.

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One of Ms Gunthorpe's friends, Alara Brandie, described her as her best friend and said, "I struggle so much to put into words how much she meant to me."

Courtney Pignataro wrote in her victim impact statement that she felt "like I have been robbed of a life with my best friend".

The court heard Detective Senior Constable Melanie Macfarlane, from Victoria Police's Collision Reconstruction and Mechanical Investigation Unit, calculated the car was travelling at 116 - 144 km/h at the beginning of the tyre marks left on the road, and at impact was going 94 to 128 km/h.

But this was challenged by defence witness Dr Shane Richardson, a mechanical engineer, who told the court there were issues with how Detective Senior Constable Macfarlane arrived at her calculations.

He said the damage to the car was not consistent with such speeds, and the evidence suggested the left brake was not working.

Defence counsel Eleanor Millar said Jones had been trying to get his life back on track before his son was born, but Eli's death unravelled these efforts.

Ms Millar said rehabilitation was a sentencing consideration that was not irrelevant, given her client's young age.

She asked Judge Kevin Doyle to consider the grief Jones had suffered from losing his partner, on top of the deaths of his son and, earlier, his friend.

Ms Millar also urged the judge to take into account the injuries Jones sustained in the crash.

She said her client had braked and tried to avoid a crash.

"It's not a case of competitive driving or showing off to friends or attempting to evade police, as is often the case with other young offenders," Ms Millar said.

The plea hearing continues on Wednesday.

Note: The Bendigo Advertiser has previously reported Jones' surname as Hogan-Jones, in accordance with court documents. However, defence counsel have since clarified that his surname is Jones. This article has been amended to reflect this.

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