Serial killer Kathleen Folbigg has always maintained her innocence. This week, a judge 'reinforced' her guilt.

With AAP.

When the inquiry into Kathleen Folbigg’s murder conviction was announced in August 2018, she was optimistic. “Absolutely delighted”, in fact, according to a report by the ABC.

Confined behind the towering fences of Cessnock Correctional Centre, Kathleen saw the inquiry as a chance for redemption. A chance to shake off the title of Australia’s most deadly female serial killer. A chance for ‘justice’.

But this week, Former Chief Judge of the NSW District Court, Reginald Blanch, ruled that justice had already been done when the Hunter Valley woman was jailed 16 years ago.

Blanch, who conducted the inquiry, said in Monday’s report that he had “no reasonable doubt” that Kathleen Folbigg was responsible for the murders of three of her children — Sarah, Patrick and Laura, aged eight months to 19 months — and the manslaughter of her first-born, Caleb, aged just 19 days.

The children all died within a 10-year period from 1989, each one before the birth of the next.

Kathleen was convicted in 2003 and sentenced to 40 years in prison, including 30 years non-parole, which was later reduced to 25 years on appeal.

The 52-year-old continues to profess her innocence, and claims that her babies died of natural causes.

When SIDS became murder.

Caleb Folbigg was born on February 1, 1989, he was full-term, healthy. But early on, he was diagnosed with laryngomalacia — “floppy larynx” — which slightly affected his breathing. The condition was mild, according to court documents, and the paediatrician said he would soon grow out of it.

On February 20, Kathleen put Caleb to bed. At some time shortly before 3.00 a.m., her husband, Craig, woke to “screamed words” and ran into the adjoining room to find Kathleen standing at the end of the bassinet, yelling, “My baby, there’s something wrong with my baby”. Caleb had stopped breathing. Ambulance officers arrived and continued CPR, but the baby, just 19 days old, was already dead.

Caleb’s death was declared to be Sudden Infant Death Syndrome — the term for a sudden, unexpected and unexplained death of an apparently well baby.

Two years later, on February 13, 1991, their second child, Patrick, too died suddenly in his bassinet. The eight-month-old's death was attributed to unexplained cardiac arrest. Just three months earlier, he'd survived an incident of respiratory distress.

Sarah, aged nine months, died on August 29, 1993, during the night of unknown natural causes, despite consultations from a SIDS specialist.

Laura died on March 1, 1997, at 19 months. Once again, the cause was undetermined.

Then, a grieving Craig Folbigg discovered one of Kathleen's diaries.

Kathleen Folbigg's damning diaries.

Police ultimately discovered several books filled with Kathleen Folbigg's writing. The majority was mundane, the scribblings of an apparently exhausted mother chronicling her daily life and perceptions of parenting. "[The diaries were me] trying to figure out things," Kathleen told ABC's Australian Story in 2018, "Understand the un-understandable."

But to prosecutors, they were alarming.

In an entry while pregnant with her daughter Laura in January 1997, Kathleen wrote: “This time I am going to call for help, this time I’ll not attempt to do everything myself any more. I know that that was my main reason for all my stress before and stress made me do terrible things…”

In November that same year, Kathleen wrote: "With Sarah, all I wanted was her to shut up, and one day she did".

A month later, she wrote, "Laura's a fairly good-natured baby, thank goodness. It has saved her from the fate of her siblings. I think she was warned".

Kathleen was charged with five counts of murder. During her seven-week trial, the prosecution argued that she had smothered her children to death.

A jury declared her guilty of four counts of murder and one of manslaughter on May 21, 2003. She was sentenced that October.

The inquiry.

In 2015, lawyers for Kathleen lodged a petition casting doubt on some of the evidence that led to her conviction.

Three years later, NSW Attorney-General Mark Speakman announced an inquiry, saying he'd formed the view it was necessary "to ensure public confidence in the administration of justice".

The $2.4 million, year-long probe saw Kathleen front Lidcombe Coroner’s Court for questioning over her diaries, some of which she was accused of disposing.

"I don't know why any of my children died, but I didn't kill them," she told the court during the inquiry. "I didn't kill my children. And these diaries are a record of just how depressed... and [what] a struggle I was having."

"The evidence pointed to no person other than Ms Folbigg."

Reginald Blanch's final report on Monday acknowledged that while medical evidence alone "neither proves nor disproves that any of the children were smothered",  the inquiry "failed to identify a reasonable natural explanation" for the children's deaths.

"The investigations of the Inquiry have instead produced evidence that reinforces Ms Folbigg’s guilt," the report read.

"The only conclusion reasonably open is that somebody intentionally caused harm to the children, and smothering was the obvious method.

"The evidence pointed to no person other than Ms Folbigg."

In a response to the report, Kathleen's lawyer, Stuart Gray, said they were disappointed, but looked forward to NSW Governor Margaret Beazley's consideration of the transcript and report.

"Ultimately, it is a matter for the governor to dispose of Ms Folbigg's petition," he said in a statement. "We remain hopeful that consideration will be given to the evidence of the various experts that appeared at the inquiry and those that submitted reports after it."

Attorney-General Speakman said he will recommend to the governor that no further action be taken.

He also said he'd spoken with Craig Folbigg about the report.

"I acknowledge that the decision to commence an inquiry has further aggravated what already was an unimaginable tragedy," Speakman said in a statement.

"I am sorry for the toll that the inquiry has taken on Mr Folbigg and family members over the last year."

He said he hoped the findings "might provide comfort in some way to the relatives of Caleb, Patrick, Sarah and Laura, and will dispel community concern regarding Ms Folbigg's convictions".

Kathleen is eligible for parole in April 2028.