The now 27-year-old man, referred to by the court-ordered pseudonym 'ZT' as he was a juvenile at the time of the alleged offences, faced the first day of his NSW Supreme Court murder trial in Wagga on Tuesday.

ZT has been accused being part of an extended joint criminal enterprise with another man who cannot be identified for legal reasons, then aged 39, to murder Mr Chaplin and dispose of his body via burning and burial in vacant land near Gerogery's Main Street.

ZT's defence barrister, David Dalton, told jurors that he did not participate in Mr Chaplin's murder and had "fallen under the spell" of the other man, a "diabolical" older person who had supplied him with illegal drugs.

In his 43-minute opening address, Crown Prosecutor Paul Kerr said the other man had inferred a "strong motive" for the killing as he caught Mr Chaplin sexually abusing an infant.

Mr Kerr said given the other man was in prison for sexually abusing two children, it was more likely Mr Chaplin had caught the man offending.

"Would you kill somebody to cover up a crime you had committed? A very serious crime, an awful crime. And if you would, would you get one of your mates to help you? It sounds like a plug for a mystery novel, but it is real," Mr Kerr said.

Police unearthed parts of Mr Chaplin's skeleton at Gerogery during a search in September 2019.

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Jurors heard Mr Chaplin was not reported missing and police had no idea about his death until the other man told a fellow prisoner that he had killed somebody, which was reported to a guard.

Mr Kerr said the other man had enlisted ZT to help murder Mr Chaplin and then dispose of his body with the help of the other man's then partner, to conceal his child sex offending.

"Under no circumstances could William Chaplin be allowed to go to police, and the longer he was alive, the greater the risk; William Chaplin had to be silenced," Mr Kerr said.

Mr Kerr said the prosecutors had a "circumstantial case" as there was no direct evidence for who killed Mr Chaplin or how, or exactly when during May 2010.

In a 15-minute address, Mr Dalton told jurors there was no independent testimony to suggest ZT had taken part in the murder and he had concealed the crime "out of fear for his life" after being befriended at age 12 by the other man, who was 23 years his senior. Mr Dalton said ZT was "scrambled" by drug addiction and the trauma of seeing, at age 16, the other man murder Mr Chaplin, then aged 25.

"A major issue for you in this trial to determine whether things [ZT] has said are reliable. If he is not reliable, then he is not guilty, because that is all the prosecution has," Mr Dalton said.

"[ZT] said he killed Mr Chaplin in ways that cannot be true, in ways that are contradicted by the evidence."

The trial continues on Wednesday.

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