A teenager who killed Bridgette 'Biddy' Porter, 10, had repeatedly asked for help with her declining mental health, but the only person who took her seriously was her grandmother.

The 14-year-old, known as XR for legal reasons, was in the throes of psychosis when she stabbed Biddy to death at a property in Gunnedah, in north-east NSW, on July 8, 2020.

XR confessed to the killing shortly afterwards, but the NSW Supreme Court found she was not legally responsible due to her then-undiagnosed schizophrenia. She remains in a mental health facility.

An inquest into Biddy's death began on Tuesday, examining whether systemic failures contributed to the tragedy or whether XR's mental health issues could have been detected sooner. An urn with Biddy's ashes was in court throughout the hearing.

The inquest heard from a range of witnesses, including XR's parents, her GP, and a representative from the NSW Department of Education - all of whom said her behaviour did not seem worrying enough to warrant intervention.

Her behaviour included killing six chickens and bringing body parts to school, punching a boy in the head, naming her favourite knives, self-harming, talking about suicide, writing 'violent killing stories', and saying she heard voices and saw eyes.

XR's maternal grandmother appeared to be the only person gravely concerned about the teenager's behaviour before the killing, but the inquest heard XR's mother and father 'shut' her out for 'parenting' their daughter.

Biddy's father, Dominic Porter, also told the inquest that he tried to take his own life twice so he could be with his daughter, in a statement so moving that even the state coroner was fighting back tears.

In the year before she stabbed Biddy to death, XR had asked her father about the voices she was hearing and the eyes she was seeing, but he dismissed her concerns as 'cr*p' from her imagination because she was 'looking for attention'.

He assumed she was being bullied and blamed her overactive imagination on her maternal grandparents for allowing his daughters to watch 'inappropriate' films. 

The father told the inquest that he taught his two daughters to 'whack' children who gave them trouble at school, and he didn't think the chicken incident was concerning because XR grew up on a farm, as he did, and death was normal.

He wasn't worried when XR told her mother about her killing fantasies, or about how she had actually wanted to kill her parents instead of her sister, because - at that stage - she wasn't acting on her homicidal fantasies.

XR's mother told the inquiry she still has no idea why the chicken incident was 'such a big red flag' and, like XR's father, didn't understand why her daughter's interest in knives was an issue.

These included a paring knife, a pocket knife, a filleting knife, and a homemade knife, which she named Rue, Storm, Ray, and Lou.

When questioned on the knives, XR's mother said: 'My juicer's name is Juicy, is that a red flag?'

Both parents described XR's maternal grandmother as someone who 'likes to tell stories' and was 'making assumptions' about the teenager.

But evidence given at the inquest showed XR's grandmother was the only person who recognised the severity of the situation.

XR's grandmother told the inquest that she saw a lot of red flags in her granddaughter's behaviour - particularly when she heard about the chickens.

She told her daughter, XR's mother, that XR appeared to be 'mentally disturbed', but was repeatedly told by both parents to stop making inquiries.

The grandmother said she and her husband, XR's grandfather, spent an hour speaking with their granddaughter about the chicken incident.

She told XR's mother that the chicken incident was 'not normal' and that XR's problems needed to be dealt with by a specialist. 

'XR was a very clever little girl and she talked her way out of things, and this is how she manipulated her parents, and that's what I said to my daughter,' she told the inquest.

'I said, "You can't trust what she's saying to you, you need someone smarter than her".'

The inquest heard the grandmother had suggested to XR's mother that she take her out of school and seek professional help.

When XR's mother was asked about the grandmother's involvement, she appeared irritated that her mother had 'made assumptions'.

Biddy's relatives made statements at the end of the inquest, including her parents, grandparents, and her aunts.

When her father got up to make his statement, several family members gathered around the witness box to support him.

Through tears, Mr Porter said: 'Biddy is not a file or a case number, she is not the subject of evidence - she's my daughter,' he said.

'She was a young girl with her whole life in front of her. She mattered, she still matters.'

He said Biddy was a gifted writer who dreamed of being a journalist, before talking about the toll her death took on his family and his mental health.

'In October 2024, I tried to take my life two days running. I saw no hope and no future, I just wanted to be with Biddy - I felt the same from the killing [in 2020] until that point,' he said.

'I was going to kill myself by whatever means to get to Biddy quicker.'

He told the inquest there was an open casket at Biddy's funeral, but he wasn't allowed to touch her because her fragile body would have 'fallen apart'.

Mr Porter said there were 'obvious' warning signs that XR's parents missed, and that he hoped the inquest would prevent a tragedy like that happening again.

At the end, Magistrate Teresa O'Sullivan was struggling to hold back tears, along with several others in the room, including legal professionals.

Ms O'Sullivan said it had been a 'privilege to get to know Biddy' through her photos, writing, artworks, and family statements.

Biddy's parents separated after her death. Her mother remarried.

Oral submissions will be heard in November.