On June 21, Liam McAtasney was found guilty of murdering aspiring artist Sarah Stern, 19, inside her Neptune City house in December 2016.

She was strangled and thrown over a bridge in order for McAtasney to steal her inheritance money.

Police became suspicious of him during their investigation, but lacked substantial evidence linking him to the crime, news.com.au reports.

Two months after Sarah's disappearance, police received a tip off that let them catch his chilling confession on tape, sealing McAtasney's fate.

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The chilling confession — secretly recorded by a friend of McAtasney's — was heard in court, and is set to air on this week's episode of true crime series Injustice with Nancy Grace.

"I thought I was walking out with 50 G's, 100 G's," McAtasney complained to his former friend, horror filmmaker Anthony Curry, in the audio.

Michael Stern was widowed three years ago before he lost his only child, Sarah. Photo / Supplied
Michael Stern was widowed three years ago before he lost his only child, Sarah. Photo / Supplied

What he didn't know was that Mr Curry had given police permission to bug his car, allowing them to eavesdrop on the conversation.

The young filmmaker began working with police after telling them McAtasney had pitched a bizarre idea for a film prior to Sarah's disappearance. Police encouraged him to meet with McAtasney and record it. Portions of the recording are featured in Episode 5 of Injustice With Nancy Grace.

After complaining that she only had $10,000 in cash, McAtasney went on to say, "And this money — I dunno if it was burnt or something … all the money was terrible quality. I don't even know if I can put any of it in the bank."

He then eagerly detailed how he killed Sarah, without hesitation.

"I pretty much hung her — like, I picked her up and had her dangling off the ground," he said to his friend. "She pissed herself and said my name, and that was it."

After "choking her out," McAtasney said on the video, Sarah was just "laying there, like, having a seizure for a couple minutes," before he stuffed a shirt down her throat to keep her from vomiting.

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Preston Taylor was offered a plea deal if he agreed to testify against McAtasney. Photo / Supplied
Preston Taylor was offered a plea deal if he agreed to testify against McAtasney. Photo / Supplied

In the middle of explaining how long it took for Sarah to die — 30 minutes — McAtasney began musing about her dog's behaviour during the incident, while police listened in.

"Her dog laid there and watched as I killed her," McAtasney said in the recording. "It didn't do anything … What kinda dog is that? It looked like a beagle, but it sounds like a Great Dane."

Anthony Curry's courage in meeting with his former friend — and testifying in court — led to the successful prosecution of both McAtasney and his former roommate, Preston Taylor — who once took Sarah to prom and helped cover up the crime.

Liam McAtasney allegedly killed Sarah Stern in order to steal her inheritance money. Photo / Supplied
Liam McAtasney allegedly killed Sarah Stern in order to steal her inheritance money. Photo / Supplied

Facing up to 20 years in prison, Taylor testified against McAtasney after accepting a plea deal. Taylor told the court he drove Sarah's car up to a bridge above Shark River on McAtasney's instructions, before they both pushed her body over the bridge to make it look like a suicide.

McAtasney has maintained his innocence and declined to testify in his month-long trial. The now 21-year-old was sentenced to life in prison without parole in June this year.