David Collisson, 53, was reported missing on October 15 after he was last seen at a property in Menah, northwest of Mudgee in NSW.
NSW Homicide Squad detectives have launched a search of the rural property with the help of police divers and dogs as part of their investigation.
Local police referred the case to homicide investigators after Collisson could not be located or contacted.
His suspicious disappearance comes more than two decades after he was convicted of the "cold blooded, callous and cruel shooting" of 17-year-old Shahab Kargarian.
Shahab and his girlfriend, Shabnam Faiz, 17, were eating takeaway in a park on Sydney's north shore in 2000 when Collisson, accompanied by another man, demanded the teen's wallet.
Collisson then shot the victim four to five times in a robbery gone wrong.
His sentencing judge, Justice Tony Whealy, said the killer was "determined to end this young man's life and kept firing until he had achieved his object".
"He ... has simply failed to face up to and take responsibility for his actions," Justice Whealy said.
Collisson pleaded not guilty to the murder and maintained his innocence throughout the trial and sentencing.
A former able seaman, Collisson worked as a general manager at a car dealership prior to Shahab's murder.
He was sentenced to a minimum of 18 years in prison with a potential parole date of October 7, 2018.
Police urged anyone with knowledge of Collisson's whereabouts to contact Crime Stoppers.
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