A Hunter Valley woman, diagnosed with what was once know as multiple-personality disorder, has been sworn in as different witnesses while giving evidence in a child sexual abuse trial.

This article contains content some readers may find distressing.

The case is the first of its kind in Australia, where an alter is giving evidence.

There was a case in 2019 where another woman had permission to give evidence as a different personality, but she never got to because her father pleaded guilty mid-trial before any alters were called.

A man, who can only be known as SN, has pleaded not guilty in Newcastle District Court to sexually abusing two of his children over a seven-year period, ending in 2016.

The jury has been told one of the children, LN, has been diagnosed with dissociative identity disorder.  

LN was sworn in as herself when she entered the witness box on Friday.

Within 15 minutes she had switched identities, becoming a 13-year-old girl.

Her voice changed from calm and measured to a girlish and sometimes brash tone.

The names of different personalities are often referred to as alters, but LN told the jury she preferred the term "system members".

Judge Ian Bourke treated the second personality as a different witness, making her take a fresh oath to tell the truth.

Re-sworn as teenage girl

"I don't lie, I promise I don't lie," she told the court.

The change in personality came about after Crown prosecutor Kristy Mulley asked permission to speak to the girl.

"OK, I am not going to be rude," LN replied.

Her tone changed to one that was loud and angry when asked about the detective first assigned to the case.

"She didn't do a very good job, she didn't complete my statement," she said.

LN, speaking as the teenage girl, told the court about other officers getting involved.

She said she was reluctant to talk, fearing she would be "killed". 

She described mistreatment she allegedly experienced as the 13 year old girl. 

"[I was] locked in a room, they wouldn't let me out, I didn't know what to do," she said.

Her voice lowered to a whisper when she began talking about the alleged sexual abuse.

"Dad was making my sister and I look at magazines in the bathroom … and [the people] were naked," she said.

"Then he got my sister and I to start touching him down there.

She said her dad wanted her and her sister to come up with stories about what they liked about the books.

"I didn't like anything … really bad things happened and they got worse," she said raising her voice.

In her opening address on Tuesday, Crown prosecutor Kristy Mulley told the court LN had multiple identities but not all would give evidence.

LN is expected to give evidence over several days.

What is dissociative identity disorder?

Trauma and dissociative identity disorder expert psychiatrist George Blair-West gave evidence on Wednesday.

Dr Blair-West told the court the disorder was not a mental illness but instead a "powerful reaction" of the human mind.

"There is nothing wrong with LN's brain … she just developed from a very young age an incredibly powerful defence system against betrayal trauma," he said.

"Betrayal trauma is when people who are supposed to care for you and protect you are the perpetrator of the trauma."

Psychologist Dianna Kenny testified on Thursday, contradicting Dr Blair-West's evidence.

Professor Kenny said dissociative identity disorder was a severe mental illness.

"It is about as severe as they get," she said.

The trial is expected to run for up to eight weeks.