"I knew this guy was the devil."

Colleen Gwynne will never forget coming face to face with Bradley John Murdoch for the first time, while investigating the murder of Peter Falconio.

"I wanted to cry but I wasn't going to let him beat me," said the former policewoman, who is now the Northern Territory Children's Commissioner.

Fifteen years ago during the initial stages of the Falconio case, Ms Gwynne's first instinct was to distance herself.

"There was nothing about what the NT police were doing at the time that was actually very flattering or made us look like a group of people who knew what we were doing," she says.

"So what I did was try to distance myself in a very small town as much as I could from anything to do with what is now known as the Falconio case."

This all changed when she got a call one night from the police commissioner, who said: "The case is yours."

From Ms Gwynne's perspective that marked the beginning of the end of her police career.

She explains that while she realised what an opportunity the investigation presented, she was patently clear about the magnitude and complexity of the case and admits she was not feeling very confident.

The investigation

Colleen Gwynne was the lead investigator on the Peter Falconio case. ( Supplied )

To deal with those anxieties, Ms Gwynne worked day and night and read everything about the case she possibly could. Then she started her planning.

Initially, she observed the team she was working with and identified three individuals who would be the key players in solving the crime.

"There was the old-style cop who'd probably lived in Alice Springs his whole life. He was smart he was methodical but he believed something bad had occurred," she said.

"The second person was a young detective who worked in Alice Springs and she wanted a challenge.

"The third person, who ended up being best on ground, was an intel officer who was OCD like you wouldn't believe."

To begin the investigation, Ms Gwynne and her team returned to the scene.

She got her team to drop her off at the site where Falconio's girlfriend Joanne Lees hid — a salt bush in the middle of nowhere outside Alice Springs.

Ms Gwynne sat there behind that salt bush as her team drove away and left her alone. She says she has never been more terrified or vulnerable in her life.

"I wanted to cry. It was the most scary feeling. I could hear my own heartbeat — and I can actually ring someone to pick me up, she couldn't do that," she said.

"That's when it felt really real for me. I understood what this woman had been through and it was scary."

'We'll find who did this'

The next step of the investigation was to meet the surviving victim so Ms Gwynne travelled to the UK to meet Ms Lees.

At this stage, Ms Lees had no trust in the NT Police who she thought had no clue how to solve the crime.

English backpacker Peter Falconio with his girlfriend Joanne Lees. ( AAP )

After an intense, 12-hour interview with Ms Lees — who Ms Gwynne describes as an amazing witness with an unbelievable recollection of events that she told with tears, laughter and fear — Ms Lees took the first steps to restoring some faith in the investigating team.

The next trip for Ms Gwynne was to the north of England to meet the Falconio family.

At this meeting she made a promise to Peter Falconio's grief-stricken mother, Joan, that her team would find out who killed her son.

"She hugged me and from that moment we had a bond and when I left, the pressure on me and the weight on me was enormous," Ms Gwynne said.

"And the hope they had and the way they looked at me that I was the one that was going to change this for them and if I couldn't find Peter they wanted the person responsible found and convicted."

Pursuit of Murdoch

Six months later, back in Australia, at a time in the investigation where there were 2,500 persons of interest in that crime, there was a breakthrough.

A man reported to police he knew who did it and that it was a guy called Bradley John Murdoch.

The information he gave Ms Gwynne's team was so good they knew they had their first really good lead.

As soon as they could, the police pounced on Murdoch and got a sample for DNA analysis, which was a game-changer in the investigation.

"We now know who touched her on her shirt, we now know who touched the gear stick in the Kombi and we now know who made the manacles," Ms Gwynne said.

"It was Bradley John Murdoch.

"The feeling that I had at that time, I don't know if I can ever explain."

But Murdoch knew the police were onto him and panicked and escaped to the bush. They knew they had a window of opportunity which was closing every minute he spent on the run.

"If he disappeared, we wouldn't have found him — he was an expert in disguise and he was an expert in not being found," Ms Gwynne said.

"This man knows the bush like no other."

They did find him very soon after, in South Australia, but only after he committed crimes on a mother and her daughter.

When he was found he was transported to Yatala prison in Adelaide where Ms Gwynne came face to face with him for the first time.

Beating 'the devil'

"This was something I dreamed of for three years," she said.

"We arrived at the prison, the media was everywhere outside. I walked through the three sets of secure doors, my heart is beating like you wouldn't believe, I finally got to meet this man."

Ms Gwynne was not just overcome with the intensity of meeting the murderer of an investigation that had consumed her life for three years, but also with a surprising and personal connection to her past that saw her having to battle demons she had not anticipated.

"I met him and he seemed like my father," she said.

"There was a remarkable resemblance between my father, who was the most intimidating and violent man who I've ever had anything to do with and made my upbringing a nightmare.

"My past flashed before me and at the same time he was playing a game of intimidation with me.

"He stood over me, this tall intimidating figure and I was so small under his frame.

"He was yelling at me, he was spitting on my face and I was never going to take a backwards step.

"And I didn't. I played the game and I won. He took the backwards step."

Hair elastic convicts Murdoch

The item that ultimately convicted Murdoch was a small, non-descript everyday item, an elastic hair-tie.

The discovery of this item during the investigation confirmed for Ms Gwynne her carefully calculated team selection had paid off.

The officer she had described as the OCD individual was meticulous in trawling through the thousands of Murdoch's belongings confiscated as evidence from his car and trailer.

The officer went through every detail and what she found among those belongings was a hair-tie that was taken from Ms Lees when she struggled to survive at the hands of Murdoch.

"He probably didn't know how significant the hair-tie was and had it wrapped around his holster inside his belongings," Ms Gwynne said.

"I think it was a trophy but no-one will ever know."

When it was presented in evidence, Ms Gwynne says he recoiled back and would not touch it.

They knew that was it; that was the nail in his coffin.

Conviction and retribution

Bradley John Murdoch was being taken to the Darwin Police Watchhouse to be formally charged with Falconio's murder. ( AAP )

When Murdoch was convicted, Ms Gwynne was sitting in the front row.

When the jury delivered their decision she did not know how to feel.

She wanted to cry, she wanted to laugh, she felt numb.

She looked to her left and there was Joan Falconio, who mouthed "thank you" to her.

"And I had to remove myself from the courtroom and find somewhere where I could just cry and cry and cry," Ms Gwynne said.

It was not just the enormity of Murdoch's conviction that struck her, it was also the realisation this would be her last case.

"That was the moment that would mark the end of my police career," she said.

"I would never equal such a challenge. There was nothing left after that."

Hear the full story on ABC Radio's new podcast Tall Tales and True. Subscribe in iTunes, the ABC Radio app or your favourite podcasting app.