A jury found Stuart Cowan guilty of a charge of conspiracy to murder after trial in Wollongong District Court earlier this year.

The Crown argued that Cowan had conspired with his incarcerated brother-in-law, Mark Jenkin, to kill the female witness to prevent her giving evidence against him (Jenkin) in the trial of slain Mangerton pensioner Mark Dower.

Mr Dower's decomposing body was found in a surfboard bag in the shared laundry of a NSW Housing unit complex on Crana Place on April 16, 2015.

The court heard while in custody awaiting trial, Jenkin spoke to Cowan on a contraband phone about how the woman had given a statement to police implicating him in Mr Dower's death.

The woman told officers Jenkin, who was known to stand over people, was responsible for killing Mr Dower and she'd helped him dispose of the body out of fear of what he might do to her.

The Crown presented evidence during Cowan's trial of a recorded conversation in which Jenkin directed him to give the woman a lethal overdose of heroin, known as a "hot shot". Cowan then tried to find the woman and source the drugs, the court heard.

Cowan was arrested in November 2015 and has been behind bars ever since.

He will be required to serve a six-and-a-half year non-parole period before becoming eligible for release in May 2022.

Meanwhile, Jenkin has since been found guilty of Mr Dower's manslaughter and conspiring to kill the female witness. He has been sentenced to 19 years' behind bars.