The family of slain Sydney mum Monika Chetty has fought back tears during a fresh plea for any information about her agonising death from chemical burns four years ago.

The 39-year-old mother of three suffered for at least five days before she was found and was in “extreme pain”, say police.

She died at Concord Hospital in January 2014, 28 days after she was found in bushland near West Hoxton suffering acid burns to 80 percent of her body.

Detectives now believe Ms Chetty may have known her attacker and was too frightened to seek medical treatment or report the assault.

Ms Chetty's family joined police in a renewed appeal for information today.

Detective Chief Inspector Dean Johnstone, of Liverpool police, said Ms Chetty suffered the burns at least five days before she was found.

"There is no doubt that she was in extreme pain for all of that time, and she had also gone without any kind of pain management," he said.

Ms Chetty, a qualified nurse, was homeless at the time of her attack.

Before her death, she told police she suffered the burns at Bigge Park, which officers have since ruled out.

Det Insp Johnston said she did that to protect herself and loved ones from any further attacks.

"In my career as a NSW Police officer, after nearly two decades of service, I've never seen injuries of this nature," he said.

"This is a personal crime, committed against a 39-year-old woman.

"She was estranged from her family at the time, but still had a family and people who cared about her."

Ms Chetty's son Daniel burst into tears as he spoke about the search for his mother's killer.

"My mum was a very loving, caring, happy, bubbly woman who loved every one of us," he said.

"This has left us scared and wanting answers for why this happened.

"It has been very painful for our family. We need someone to come forward and tell us why."

Det Insp Johnton himself fought back tears as he strongly urged anyone with information to come forward.

"So justice can be served for Monika and her family".