It is a true crime story which has shocked the world.

We may never have heard 'The Teacher's Pet' if Nicholas Cowdery had taken his two chances to prosecute Lynette Dawson's husband Chris Dawson.

The mother-of-two abruptly vanished from her Sydney Northern Beaches home almost 40 years ago in January 1982.

Her daughter, Shanelle, was only four-years-old at the time. Broken-hearted and alone, she avoided reporters for decades and refused to condemn her father amid the speculation about who killed her mother.

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The case sparked renewed global interest after she was featured as one of the main subjects in a podcast called 'The Teacher's Pet', produced by The Australian newspaper.

More than 36 years after Ms Dawson mysteriously vanished, the man who decided not to prosecute Mr Dawson on two occasions for her death has revealed why he didn't lay charges.

Former NSW Director of Public Prosecutions Nicholas Cowdery QC told ABC, the evidence simply wasn't strong enough.

"Without a body, without knowing first of all whether in fact she is dead, without knowing secondly if she is dead, how she died, it's very hard to mount a case of a reasonable prospect of conviction just on motive and the undefined existence of means and opportunity. That makes it very weak," he said.

"Lyn Dawson disappeared, and that really is as far as I can take it in my own mind."

It comes after an explosive 60 Minutes interview, in which Shanelle finally shared her side of the story.

WHAT HAPPENED TO LYNETTE DAWSON?

Lyn and Chris Dawson were childhood sweethearts.

On the surface, they had it all — a wholesome relationship, respectable jobs and a home in an idyllic suburb on Sydney's northern beaches.

Lyn worked a nurse, and was seen as a kind and gentle soul. Chris was a popular high school PE teacher and talented sportsman.

They were married for 12 years, and had two daughters — Shanelle and Sherrin.

But their relationship wasn't as picture-perfect as it appeared.

In 1980, Chris began a relationship with his 16-year-old Cromer High School student Joanne Curtis. Their relationship became sexual, and Chris employed the girl as a babysitter for his two daughters.

Joanne Curtis pictured with Chris and Lyn's two daughters, Shanelle and Sherrin. Photo / Channel 9
Joanne Curtis pictured with Chris and Lyn's two daughters, Shanelle and Sherrin. Photo / Channel 9

He tried convincing his wife to let the girl move into their home for the remainder of her schooling, saying she came from a broken home and had a violent stepfather. The couple ended up in marriage counselling.

On January 9, 1982, Lyn was due to meet her mother at Northbridge Baths, where Chris worked.

But Lyn never arrived. Chris later claimed he dropped her off at a bus stop in Mona Vale, and that she called him later in the day saying she needed time to herself.

No more than two days later, Joanne had moved into Chris' home — and marital bed. She would even wear Lyn's jewellery and her clothes.

Chris didn't report his wife as missing until six weeks later, and told everyone she had run off to join a religious cult.

Over the following years he divorced his missing wife and ended up marrying Joanne. They moved to Queensland and had a child together.

In 1990, the pair split up. According to Hedley Thomas, the senior News Corp journalist who has investigated the case for 'The Teacher's Pet', Chris became increasingly angry and threatening towards Joanne. It was then that she went to the police, advising them to search the garden in his old Bayview property.

A body was never found — but they did discover Lyn's pink cardigan during a police dig. The garment had multiple cut marks consistent with stabbing.

Two coroners later found that Lyn was murdered by a "known person", but no charges were ever laid.

LYNETTE'S DAUGHTER SPEAKS OUT

The absence of her mother left Shanelle feeling abandoned and confused for much of her life.

"I try not to dwell in self-pity, but I guess I do sometimes dip in there, especially on Mother's Day when I see everybody out with their mums," she told 60 Minutes.

But Shanelle spoke fondly of her father, saying he was "fun". "He taught us tennis and played in the pool with us," she said. "I would say he's got a side to him that's a lot like a loveable, adorable puppy dog."

It's a perception she struggles to reconcile with the scrutiny on his potential role in her mother's murder.

The absence of her mother left Shanelle feeling abandoned and confused for much of her life. Photo / Channel 9
The absence of her mother left Shanelle feeling abandoned and confused for much of her life. Photo / Channel 9

"There is huge conflict in that. Especially people who are quite angry, and rightfully so, wanting answers about my mum. Yet there's still that part of me that really loves my dad, and feels protective of him."

She said he was a loving father to her, and struggled to see him as the narcissistic, abusive man that others made him out to be.

Her father and new stepmother never shared stories about Lyn, saying it was "like the day she left she ceased to exist".

Bev McNally — who was another student at Chris Dawson's school — tells another story. Before Joanne came along, she was the family babysitter.

"Everything in the house had to be perfect because she never wanted to upset him," Bev told 60 Minutes. "He did lose his temper quite quickly. A couple of times at their house I did see him turn quite nasty, and it shocked me."

"The first time Chris and Lynette were both in the kitchen, and Chris went to get a glass out of the cupboard. The glass was dirty, so he ranted at Lyn for putting a dirty glass in the cupboard. He grabbed the tea towel. I thought he was going to wipe the glass with it, but he didn't. He actually flicked her really, really hard right across the back with the tea towel.

"The second time I saw him being extremely violent towards her near the bedroom door and that just shattered me. And that was the one that broke it."

Bev McNally - who babysat the Dawson children before Joanne - described Chris' behaviour as nasty and abusive. Photo / Channel 9
Bev McNally - who babysat the Dawson children before Joanne - described Chris' behaviour as nasty and abusive. Photo / Channel 9

She said she never went back to the house again.

Bev firmly believes Lyn did not choose to leave. "She would never have left those children on her own accord," she said. "They were everything to her. Everything."

Shanelle accepts that the evidence is stacked against her father. "My emotions and my love for my dad aside, I can see that there a lot of people saying all those things, it's not just one person," she said.

Does she think her father is a murderer?

Shanelle can't say yes or no. She said her father looked her in the eye and swore he never, ever hurt her mother.

She desperately wants to believe him.