Sydney man allegedly confessed to having abducted and killed a schoolgirl in 1998

A Sydney man repeatedly confessed to having abducted and killed a schoolgirl in 1998, describing it as a stupid, botched ransom attempt, a jury has been told.

But Vinzent Tarantino's lawyer said he didn't make admissions to two of those witnesses and made false confessions to others because he feared for his life and the lives of his loved ones.

The 52-year-old has pleaded not guilty in the NSW Supreme Court to murdering Quanne Diec, 12, who vanished on July 27 in 1998 after leaving her Granville home to walk to the train station on her way to school.

Her body has never been found.

"I expect the evidence will show the accused, then 31, took her from Factory Street in Granville to his father's home in Second Street," prosecutor Pat Barrett said in the crown opening address on Monday.

"The accused there strangled her and later disposed of her body in the bush south of Sydney.

"On the 20th of November 2016, the accused walked into Surry Hills police station and told police he wanted to confess to her murder."

Tarantino's then-girlfriend Laila Faily was expected to testify to going for a drive in a van with him to an area she believed was a national park.

She would say he stopped the car, took a wheelie bin out of the van and returned about an hour later.

He allegedly told her he had kidnapped Quanne because he wanted to get a ransom.

Tarantino also told a friend, Geoff Maurer, he had taken an Asian girl, things went horribly wrong, she was uncooperative and he "cancelled her out", Mr Barrett said.

Years later, Tarantino told another girlfriend he had killed Quanne during a botched ransom bid.

Just before he went to police in 2016, Tarantino told a brother he had been on drugs and "did a horrible thing".

"I killed a kid Alan. I am f***ed," Tarantino said.

He told police it had been "a stupid ransom attempt" which just went wrong.

But Tarantino's barrister, Belinda Rigg SC, said the defence disputed claims he made admissions to Ms Faily and Mr Maurer, submitting they were witnesses "who are deliberately misleading you".

She described what he told police and others as "false confessions" made because he believed he needed to do so to save his own life and his loved ones' lives.

The jury would hear evidence of Tarantino becoming paranoid about perceived threats and of there being numerous incidents involving police and psychiatric hospitals relating to his perceptions.

Tarantino did use a borrowed white van on the day Quanne went missing, but Ms Rigg said the vehicle had nothing to do with her disappearance.

She referred to a Vietnam veteran, who had been working at a mail centre which was on the route Quanne took to the station.

He had expressed hatred towards Asians, told people he had a sexual interest in young girls and went on leave just after Quanne disappeared, Ms Rigg said.

The trial continues before Justice Robert Beech-Jones.