Kathleen Folbigg is one of Australia’s most contentious and divisive figures - a mother convicted of killing her four children only to be sensationally freed 20 years later.

The 56-year-old received an unconditional pardon and was released from jail in June after an inquiry heard of new scientific evidence.

WATCH THE VIDEO ABOVE: 7NEWS Spotlight: Kathleen Folbigg sits down with Natalie Barr.

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This led to reasonable doubt of Folbigg’s guilt over the deaths of Caleb, Patrick, Sarah and Laura - who died between the ages of 19 days and 19 months from 1989 to 1999.

Now, in a groundbreaking new interview with 7NEWS Spotlight, airing on Sunday, October 15 at 7pm on Seven and 7plus, Folbigg and her best friend Tracy Chapman reveal the graphic contents of letters sent between the two after Folbigg was convicted.

During her trial, the court heard Folbigg’s diary entries, supplied to police by her ex-husband, which formed part of the prosecution’s case.

In one she wrote, “I am my father’s daughter”.

In 1969, Folbigg’s father, Thomas Britton, was convicted of the murder of her mother, Kathleen Donovan, by stabbing her 24 times, leading prosecutors to see this as an admission of guilt.

Following the death of Laura, 19 months, Folbigg wrote, “I feel like the worst mother on this earth, scared to 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.”

She also wrote , “Heaven help the day my memories surface and I recall. That will be the day they lock me up and throw away the key.”

After hearing this in court, Chapman wrote to Folbigg in jail asking outright if she was really guilty of killing the children, and she tells Natalie Barr exactly how Folbigg explained these disturbing written entries.

Folbigg wrote back, “You’re very lucky I don’t offend too easily anymore, girl. But after being called a child killer, there’s nothing that can hurt my feelings any worse.

Folbigg added, “To start with, I’ll agree, some of my entries sound atrocious. All I can say is that at times, and at the time in question, I wasn’t in a good, positive frame of mind… the diaries were used to dump every negative emotion, feeling, thought, I’ve ever had. I didn’t use them like other people to record all the joyous happenings.”

She then said in her letter, “You must consider, I had a man in my life that didn’t concern himself with my emotions, only his own. A very self-centred man, only showing sparks of real generosity when it came to truly caring how I felt at any given moment.

“I actually grasped at anything, Trace. If it wasn’t physical, medical, or genetic, what was it? I thought I was to blame… I blamed myself. I convinced myself that I’d failed as a mother, a woman even…

“I was tortured, heart smashed, no confidence in myself at all. Felt stomped on, torn to pieces, and couldn’t gain comfort from the people I kept turning to.”

Folbigg also admitted the reference to her father was a mistake and tried to clarify her meaning.

“I am my father’s daughter’, definitely am not referring to him being a murderer. I actually asked, “Am I totally different?” Just wrote it down wrong.

“Either way, I was referring to my opinion of the man, I thought he was the biggest loser I had ever come across. His actions destroyed a life, destroyed my life, harmed many others.

“He lost me, my respect, and most of his life. Considering how I felt about myself, does anyone blame me for thinking how he cursed me? That was what I meant.”

Documented by trial records, interviews and new research, this 7NEWS Spotlight world exclusive will give an unprecedented look at one of Australia’s most controversial cases, and give viewers the chance to come to their own conclusions.

7NEWS Spotlight: Unbroken – The Kathleen Folbigg Story airs Sunday at 7pm on Channel 7 and 7plus.