Almost every day about 9.30am, Tracy Chapman's phone rings. "Kath" is the name that flashes up on the screen but before either woman can speak, a robotic American voice interjects.

"You are about to receive a phone call from an inmate at Cessnock Correctional Centre. Your conversation will be recorded and may be monitored. If you do not wish to receive this call, please hang up now."

Tracy Chapman never hangs up. "Good morning bub, how are you?" she says to her friend, convicted serial killer Kathleen Folbigg.

Since May 2003 when a New South Wales Supreme Court jury found Folbigg guilty of the murder of three of her infant children and the manslaughter of another, their conversation mostly has been banal — chitchat about Folbigg's daily routines, about the laundry or cleaning or other inmates.

But for the past three years, their conversations have increasingly focused on Folbigg's frustrations about the NSW Government's delay in considering her lawyers' petition for a judicial review of her case.

The petition, which was lodged with the NSW Governor in 2015, includes a comprehensive new report from Professor Stephen Cordner, one of Australia's pre-eminent forensic pathologists. It casts doubt on the forensic evidence that formed a major plank of the prosecution case in the trial and which, ultimately, contributed to Folbigg's conviction.

In phone calls recorded by Australian Story, Folbigg speaks publicly for the first time about her case and, in the process, gives her explanation for the incriminating diary entries that were a crucial part of the prosecution argument.

"You've got to understand that those diaries are written from a point of me always blaming myself," Folbigg says in one phone call. "I blamed myself for everything. It's just I took so much of the responsibility, because that's, as mothers, what you do."

New forensic opinion

In 2003, Folbigg was tried and convicted of the murder of her infant children Patrick, Sarah and Laura, and the manslaughter of Caleb. It was a high-profile trial that shocked the nation. She was sentenced to 40 years in jail, which was later reduced to 30 years and she is now midway through her sentence.

"I often described it, especially when I was going through the trial, as like I was just hanging onto a cliff by one finger," Folbigg now recalls.

She has always maintained her innocence, claiming that each child died of natural causes. During the trial, she opted not to give evidence. Instead her diaries became her de facto voice.

Nicholas Cowdery, who was the New South Wales Director of Public Prosecutions at the time, believes the jury got it right at the time.

"There's been an exhaustive process," he said. "The case has been assessed by a magistrate, by a jury of 12, which was unanimous in convicting, by the Court of Criminal Appeal, and the High Court has seen nothing to be concerned about in the way in which the convictions have been recorded."

In 2013, Folbigg's lawyers commissioned an extensive and detailed report from Professor Stephen Cordner, who examined the medical evidence presented at the trial.

"There is no positive forensic pathology support for the contention that any or all of these children have been killed," Professor Cordner concluded in his report.

He declined to be interviewed in case he is called upon to give evidence in future legal proceedings.

Australian Story asked another leading international forensic pathologist to offer an opinion on Professor Cordner's report.

"Fundamentally, I'm in agreement with Professor Cordner, in that all four of these child deaths could be explained by natural causes," said Associate Professor Matthew Orde, forensic pathologist at Vancouver General Hospital.

Mr Cowdery has reviewed the material and has a different view.

"I have looked at the petition that Mrs Folbigg has lodged. I've looked at the reports that have accompanied that petition. I remain of the view that the jury was correct," he said.

But he does say it is "concerning" the petition was filed three years ago and successive NSW attorneys-general have not addressed the question of whether an inquiry should be held into Folbigg's convictions. "I think this is an inordinate delay in dealing with this matter," he said.

In Cessnock Correctional Centre where Folbigg is serving her sentence, she has much time to reflect on her past. "My life just seems like it's been never-ending battles and things that I have to get over and conquer," she tells her friend on the recorded phone call.

She was three when she was placed in foster care. She has no memories of her parents but their story has been well-recorded. Kathleen's father, Thomas Britton, was a Balmain dock worker who stabbed her mother to death after a fight.

Folbigg was initially cherished by her new family but her foster sister Lea Bown says Folbigg went on to have a difficult relationship with her foster mother. "Mum was too old to have fostered her," she said. "Then my son came along and he took up all the time and her love and Kathy got none."

The repeating pattern of infant deaths

Folbigg met her husband-to-be Craig Folbigg in her teens. She was 21 when she had her first child.

"I just sort of thought that's what I was on the planet for — to meet a fella, get married, have a child, do the family. I guess because being a foster kid, a state ward, I just thought family was to be the ultimate important thing," she said.

But over a 10-year period from 1989 to 1999, Kathleen and Craig Folbigg would lose four children in very similar circumstances.

Caleb was the Folbigg's firstborn. At 19 days old, Folbigg said she found him lifeless in his cot. His death was put down to Sudden Infant Death Syndrome, or SIDS, the unexplained death of an infant. Then there was Patrick, who at four months of age suffered severe brain damage from a mysterious and unexpected "acute life-threatening event". He was revived but four months later, Patrick was also found dead by Folbigg.

The pattern repeated itself with Sarah, who in August 1993, at 10 months of age, was unable to be resuscitated. An autopsy determined it to be another SIDS death.

Then came Laura. She lived until she was 19 months old.

Once again, Kathleen Folbigg said she found her lifeless in her bed, called an ambulance, and attempted to perform CPR on her, but she was unable to be revived. It was at that stage though that the pathologist at autopsy, Dr Allan Cala, became suspicious. While he noted Laura had evidence of myocarditis, inflammation of the heart muscle, he believed it was non life-threatening and that Laura was too old to be a SIDS death.

Dr Cala shared his concerns with Singleton Detective Bernie Ryan. Folbigg's family initially believed the police investigation was a witch-hunt, but when Craig Folbigg found his wife's diaries and read their alarming contents, he changed his mind. He and Folbigg's foster sister Lea Bown became witnesses for the prosecution. Detective Inspector Ryan told Australian Story in 2004 that, "from the moment I read the diary, it was a murder investigation."

"She's a fairly good-natured baby," Folbigg wrote about Laura. "Thank goodness. It has saved her from the fate of her siblings. I think she was warned." Another entry: "I feel like the worst mother on this Earth. Scared she'll leave me now like Sarah did. I knew I was short-tempered and cruel sometimes to her, and she left. With a bit of help." And potentially most damning: "I am my father's daughter."

Natural causes of death plausible explanation: New report

The Crown case was that Folbigg had smothered her children. It relied on circumstantial evidence that she was a stressed mother prone to dark moods, that she was the one to find the dead children and did not pick up or perform CPR on the first three, and her disturbing diary entries.

Defence witnesses argued Folbigg was a loving mother and that there was no physical evidence of injuries from smothering.

Journalist Matthew Benns who sat through the trial said, "The prosecution case was very strong. There was the husband, the diaries, the police witnesses, the medical experts ... over the course of the seven-and-a-half week trial the weight of evidence built up a compelling picture".

Folbigg collapsed when the guilty verdict was read out.

Over the years, Folbigg exhausted her rights of appeal against those convictions. The NSW Court of Appeal described her diary entries as "chilling".

But a fresh forensic opinion concludes there was nothing to suggest that "any of the children have been killed".

In 2013, Newcastle barrister Isabel Reed brought together a team of lawyers, academics and students to review the Folbigg case. "What we're saying as her legal team is that the fresh evidence and expert reports that we have certainly cast doubt on Kathleen Folbigg's guilt," Ms Reed said.

They asked forensic pathologist Professor Stephen Cordner to review the medical evidence which helped convict Folbigg.

Professor Cordner's report concludes Caleb and Sarah's death were both properly categorised as SIDS deaths, Patrick's death was related to the epilepsy disorder he suffered in the last few months of his life, and that Laura's death "has been caused unexceptionally by myocarditis."

Professor Cordner's report also casts doubt on the prosecution case that Folbigg smothered the children, saying that it was surprising that in five alleged smothering events "there are no signs of smothering".

"The findings cannot rule out smothering in one or more of the cases, but especially in the case of Laura, not only is there an acceptable natural cause of death easily visible microscopically, it is important that there are neither general nor specific signs of compression of the face present," he said.

"If the convictions are to stand, they must do so without the support of forensic pathology, and in Laura's case at least, against the forensic pathology view."

Associate Professor Matthew Orde, who has reviewed the slides from Laura's autopsy concurs.

"I think this is an eminently fatal case of myocarditis. Of course we can't say for sure that this would have been the cause of death in Laura's case. All I can say is I think this provides a very good explanation for her untimely death," he said.

"I think this case certainly needs to be re-examined quite carefully."

Mr Cowdery suggests that even if it was accepted that Laura's death was due to natural causes, that does not mean that the other convictions cannot stand.

However, Folbigg's barrister, Robert Cavanagh disagrees.

"You can't just simply say, 'Well we got one wrong but the others can still stand'. They chose to run the trial and they packaged the trial — four trials happening within the one," he said.

In a statement, NSW Attorney-General Mark Speakman said he was giving "appropriate consideration" to Folbigg's petition. He said it raised "complex questions to which I have … taken extensive advice".

Mr Speakman said he expected to make an announcement in "the near future".

From her prison cell, Folbigg says she is incredibly frustrated by the delay. "We're just simply waiting for a decision. For three years now, we've been clinging to that little bit of hope."