"Australia Day will no longer be a day to celebrate for me," Ms Doyle said, holding back tears, the paper trembling between her fingers.

"It was the day Katie died."

Her daughter, Katie Louise Broadbent, 35, had been enjoying a long weekend camping with her boyfriend and their friends at the Rochester Music Festival on Australia Day this year.

In the early morning, after hours of heavy drinking and taking drugs, her boyfriend's best mate, John Forbes, 24, jumped into his 1997 blue Volvo sedan, threw it in reverse and backed over a tent.

Other festival-goers at the site, located between Bendigo and Echuca, saw the wheels spinning and heard the engine revving as the tent became tangled under his car.

But it wasn't until hours later when Ms Broadbent's boyfriend discovered her body was laying under the vehicle. His best mate, Forbes, then realised what he had done.

On Thursday, Forbes, of Westmere, pleaded guilty to one count of dangerous driving causing death, while Ms Broadbent's family read out their victim impact statements.

There were so many friends and family in attendance at the County Court of Victoria that the hearing had to be moved to the biggest courtroom available, and even then people had to be seated in additional chairs.

The crowd of more than 120 people, some who had travelled from Adelaide, cried and embraced each other as Ms Broadbent's parents, uncle, aunt and grandmother described the kind of person she had been; beautiful, vibrant, tolerant, independent. She was living in Yarraville and working as a theatre technician, but she was so much more than that, they said.

Ms Doyle spoke of a profound loss, of having to pack up her daughter's life into boxes and being unable to cope with what to do with her belongings. "We weren't only mother and daughter – we were true friends," she said.

Her father, Michael Broadbent, explained how his grief oozed out of every pore in his body. Now his pain has erupted into anger, he said.

"I feel like my heart has been crushed," he said. "It's not fair. It hurts. It hurts so much."

Adding to the family's pain is the complete lack of answers as to why it happened.

Defence barrister Peter Chadwick, QC, said his client had no memory of why he started the car or why he decided to start driving it in reverse.

"There was no need for him to leave that Sunday morning," he said. "So, the reason remains unanswered."

Mr Forbes also had support in the courtroom, with his parents and sisters sitting in the front row.

He has one previous drink-driving offence, when he was a teenager, but was otherwise described as a young man with a lot of potential who was remorseful for what had happened.

He will be sentenced on Friday morning.

- THE AGE