A decade-long investigation has made history, with Australian woman Jeni Haynes' multiple personalities granted permission to testify in court.

The landmark case is believed to be a world first, which resulted in bringing Ms Haynes' abusive father Richard Haynes to justice.

From the age of four, Ms Haynes was tortured by her father. The sickening sexual abuse occurred daily, continuing until she was 14. It was so horrific and violent, her very survival was against the odds.

But incredibly, her young mind created multiple personalities, which allowed her to cope with the physical and emotional pain she experienced. In essence, she was protected by a psychological army – 2500 different ‘alters’ who would take over her body during the abuse.

Jeni speaks to 60 Minutes reporter Liz Hayes. (60 Minutes)

"My dad inflicted, chose to inflict, severe, sadistic, violent abuse," Ms Haynes told Liz Hayes in a 60 Minutes exclusive interview.

"[The abuse] was completely unavoidable. Inescapable. And life-threatening. And he chose to do this every day of my entire childhood."

Ms Haynes – and her multiple personalities – told Hayes how her 'army' within saved her life.

Multiple Personality Disorder, also known as Dissociative Identity Disorder (DID), was once viewed with disbelief – including by the psychiatric profession.

But as psychiatrist Dr George Blair-West explained to 60 Minutes, it is now not only considered a genuine medical condition but an ingenious one.

"There is nothing wrong with Jeni’s human mind or any other person who suffers from DID," Dr Blair-West told Hayes.

"Their mind is just coming up with an incredibly sophisticated, clever solution to a scenario that most of us could not begin to understand or relate to."

Dr Blair-West has been treating Ms Haynes for over twenty years. He told 60 Minutes that when children like Jeni are subjected to significant traumas prior to the age of eight, they are able to split off parts of their mind into multiple personalities.

"The thing that seems to push the mind to do this more than anything else is realising you have no way out," he told Hayes.

"Nobody's there to save you, you're on your own, and you have to come up with a solution that is entirely of your own."

To endure the trauma of her father's abuse, Ms Haynes created personalities to cope with not just the pain, but the smells, tastes and sounds she experienced during the abuse.

Jeni Haynes and her alter personalities. (60 Minutes)

She spent years trying to report her father’s abuse and seek help, but was consistently faced with ridicule and disbelief.

But as Liz Hayes reports, it didn’t stop her. She was determined to bring her father to justice.

She spent 18 years at university, graduating with a degree in psychology, a masters in legal studies and criminal justice – and a PhD focusing on victims of crime.

Detective Sergeant Paul Stamoulis (60 Minutes)

Then finally in 2010, Ms Haynes met Detective Sergeant Paul Stamoulis who was prepared to investigate her allegations of childhood abuse by her father. She now describes Det Sgt Stamoulis as "God on legs".

"Finally someone believed me," Ms Haynes said.

"Finally somebody who was in a position that could actually impact on my dad."

Ms Haynes documented her father's repulsive crimes in an incredibly detailed 900,000 word statement that was shocking even to Det Sgt Stamoulis.

"It did take a toll," an emotional Det Sgt Stamoulis told Liz Hayes.

"A lot of people who have read that document have been seriously affected by it."

Assisting the police investigation was the fact that each of Ms Haynes' personalities lived different aspects of her trauma – meaning that she has extraordinary recall of her father’s abuse.

"She has a memory unlike yours or my memory," Dr Blair-West explained.

"Her alters are living in different time space realities, effectively. Particularly those younger ones. What Jeni does, is she can go to the part that was there then and that part will tell you exactly what happened, like it was yesterday."

Liz Hayes (60 Minutes)

What made this case more difficult was the lack of a physical crime scene. But, Det Sgt Stamoulis told Hayes, Ms Haynes' body was a crime scene. Major surgeries on her bowel, coccyx and anus were required to repair the physical damage caused by her abuse.

In 2017, 74-year-old Richard Haynes was extradited from the United Kingdom and charged with multiple counts of rape, buggery and indecent assault of his daughter.

It was agreed that the evidence in the case against Haynes would be too traumatic for a jury, and would be heard by a judge alone. Haynes pleaded not guilty to all charges.

Det Sgt Stamoulis and Dr Blair-West worked as a team to prepare Ms Haynes for her landmark appearance in court. Both agreed she should give evidence as her multiple personalities despite the challenges it presented. It would be a legal first.

Dr George Blair-West (60 Minutes)

"When you decided to give evidence, how did you decide between all of your alter personalities who was going to do and say what?" Hayes asked Ms Haynes.

"We didn’t," she laughed.

"The plan is whoever has the right answer will come out and tell it."

On February 21 this year, Symphony – a four-year old girl – was the first of Ms Haynes' personalities to take the witness stand in the NSW District Court. Symphony’s testimony lasted barely two hours before Richard Haynes crumbled. The man who destroyed Ms Haynes' young life finally pleaded guilty to his heinous crimes.

As a victim of childhood sexual abuse Ms Haynes was entitled to anonymity. She waived it, determined the world would not only hear her story, but know who her abuser was.

"I didn't want, 'A 74-year-old man was in court today'," she told 60 Minutes.

"I wanted 'Richard John Haynes was in court facing 75 charges in relation to rape, sodomy, and sexual abuse of his daughter, Jennifer Margaret Linda.' I wanted him to walk into prison with everybody knowing what he did."

Jeni Haynes' successful quest in the courts to bring her father to justice has not only validated her condition but opened the legal channels for others like her to tell their stories.