Woman wanted to be a serial killer: court

A woman who allegedly helped her former lover strangle another woman boasted about wanting to be a serial killer and described sending letters to Ivan Milat, a court has heard.

An 18-year-old woman, who cannot be named, says she was taking the drug ice and drinking with Rachael Evans, Micheal Duffy and Colleen Deborah Ayers in a Campbelltown park in Sydney's west in May 8, 2012.

Evans was reading a book about Australia's most dangerous men, featuring the serial killer Ivan Milat, the teenager told the Supreme Court on Tuesday.

"She (Evans) kept saying she used to try and send him letters in jail but they kept sending them back."

"She kept saying all she wanted to do was become a serial killer."

Evans has pleaded guilty to murdering Ms Ayers, 33, who was buried in a shallow grave on her parents' Picton property south-west of Sydney on May 10, 2012.

She claims her former lover Duffy helped and is set to give evidence against him at his trial.

But while Duffy admits to digging the grave, he denies helping carrying out the fatal attack.

Speaking at his trial, the teenager described the drug and alcohol fuelled days leading up to the attack.

On May 9, she said she was bullied into filming Evans, Ms Ayers and Duffy having sex at a hotel room.

Evans was dominating the scene and that they had tied Ms Ayers up with bedsheets.

"I was 16, I had never seen anything like that in my life," the teen cried.

When they arrived at Ms Ayers parents' property the following day, the group continued drinking and doing drugs.

At one point, the teenager said Evans told her that she "gave Ms Ayers 100ml of methylated spirits in a needle claiming it was ice".

Later when Evans heard Duffy and Ms Ayers having sex, the teenager said she "lost the plot".

"She (Evans) said "I'm going to do it, I'm going to do it".

The teen told the court she saw Evans go into the bedroom with a belt and heard Ms Ayers say, "please don't".

When the teenager walked into the room Duffy was kneeling naked on the bed, Evans was standing at the foot of the mattress and Ms Ayers' body was lying face down, half off.

The trial continues.