The voice of a little girl lost has rung out nearly six years after her violent death.

A montage of photographs and videos of Biddy Porter, who was killed by a teenage girl known to her on a rural NSW property in 2020, was played on Thursday afternoon at an inquest into her death.

The images showed Biddy as a happy baby, smiling proudly in her pristine school uniform, hugging her parents and cuddling her beloved dog Breeze.

One video showed her singing about stars and, in another, playfully twirling her hair.

"Oh hi," she said.

"Do you like my little curl?"

This is what her bereaved mother Rebekah Keukenmeester wanted: for Biddy's voice to be heard and her face to be seen after her case was anonymised by a court due to the age of her killer.

"I wanted the world to know her," Ms Keukenmeester told Lidcombe Coroners Court in a recorded statement.

Biddy died of multiple sharp force injuries while staying at the property in the school holidays on July 8, 2020.

The NSW Supreme Court found Biddy's killer, who cannot be identified for legal reasons, was not criminally responsible due to mental illness in 2021.

Two psychiatrists found the teenager had schizophrenia and was in acute psychosis at the time of the killing.

She has been in custody since her arrest.

Biddy's parents went public with her identity on the fourth anniversary of her death, launching a petition calling for the inquest.

The world was finally able to know Biddy as a "happy, intelligent and artistic soul", Ms Keukenmeester said.

"Over the years, I've watched her friends grow and achieve milestones, going to high school, having their first boyfriends and pursuing their dreams," she said.

"(Those) dreams and joys were stolen from my daughter, Biddy, a little girl who remains frozen in time: forever 10."

Two weeks after the 14-year-old confessed to her mother she thought about killing people "all the time", she killed Biddy while the girls were left alone in the farm house, the inquest heard.

Counsel assisting Peggy Dwyer SC outlined the teenager's descent into psychosis a year before the killing, including slaughtering six chickens on the farm, acts of self-harm and a sense of not feeling real.

The teenager's parents did not recognise the symptoms as a mental illness, but put her behaviour down to school bullying, video games and teenage hormones.

"I'm very ashamed to say I didn't believe in mental illness," the killer's mother told the inquest on Tuesday.

Biddy's father Dominic Porter said he hoped the inquest raised awareness about the signs of serious mental health to spare another family.

"Biddy, I love you, I miss you every day, I am sorry for every moment you were robbed of," Mr Porter said, through deep sobs.

"I will keep trying to make sure your name means more than just loss.

"I love you to the moon and back."

State Coroner Teresa O'Sullivan will deliver her findings at a later date.

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