Diesel mechanic Nicola Gilio was among several worried adults looking for the missing girl when he found the victim tied up in a men's toilet cubicle and Sampieri trying to leave the bathroom at a Kogarah dance studio.

The 55-year-old - who was on parole at the time for raping a woman in 2012 - had grabbed the girl as she entered the bathroom, and later dragged her into the men's toilet.

He subjected her to depraved acts for more than 40 minutes in mid-November 2018.

Mr Gilio's stomach and the back of his neck - from ear to ear - were sliced by the knife-wielding Sampieri as the men fought for several minutes.

"He had no regard for me or my family ... his only regard was a thirst for abuse," the mechanic told the NSW District Court on Thursday.

"This is the reason I can't sleep at night. The flashbacks are still there."

Sampieri has pleaded guilty to three counts of raping a child and seven other offences related to the attack.

He's also due to be sentenced over admissions he made about 94 offensive phone calls to 91 women between August and the day of the attack.

Mr Gilio told the court he could still remember the feeling of the scalpel being dragged across the back of his neck and the sight of the girl on top of the toilet with her hands, legs and neck bound.

The previously "happy-go-lucky" man is now pessimistic about the world and sees all children as vulnerable to attack from people like Sampieri.

Mr Gilio said there was no sentence harsh enough for Sampieri and wished the rapist a life of "pain, darkness and misery".

"The pain is a life sentence to all of us involved ... we live with this pain every day of our lives," he said.

"There is no forgiveness or rehabilitation for what you have done. For the consequences of your actions, you must suffer."

Upon realising what Sampieri had done to the girl, Mr Gilio screamed "Is this what you do?" and rammed Sampieri's head into a brick wall.

The rapist soon reached for his knife - used to detain the girl - and began swinging at the good Samaritan and the girl's mother who'd also entered the bathroom.

Sampieri was only overcome after the bleeding mechanic grabbed him in a headlock as dentist Jeffrey Stack burst through the door and punched the pedophile twice in the face.

Acting Judge Paul Conlon, who will eventually sentence Sampieri, on Thursday thanked Mr Gilio for his "incredible act of bravery".

Sampieri sat slumped in the dock with his head bowed for much of the hearing. He's due to face the court again on December 9.

Australian Associated Press