AAP

Two brothers have been jailed for plotting the murder of a convicted drug dealer they believed had slain their beloved grandparents.

The dismembered body of their target, who was abducted in 2001 while on work release from prison, was found in plastic packages on the banks of a northern NSW river.

In the NSW Supreme Court on Friday, Justice Derek Price jailed Anthony Perish, 42, for at least 18 years for the murder of Terry Falconer and for conspiring to kill him.

His brother, Andrew Perish, 41, was jailed for at least nine years for the conspiracy.

Anthony Perish's subordinate, Matthew Lawton, 45, was jailed for at least 15 years for the murder and the conspiracy.

Justice Price accepted the brothers' main motivation was that they believed Mr Falconer had murdered their elderly grandparents, who were shot dead at their Sydney property in 1993.

"They had become frustrated at the lack of progress in the police investigation," the judge said, adding that the brothers had been very close to their grandparents.

They were motivated "to right the wrong that Mr Falconer was perceived to have committed".

But, the judge said, a civil society could not condone their conduct.

"In our society, crime must be investigated by the police and dealt with by the courts," Justice Price said.

He found that Anthony Perish was the mastermind of a meticulously planned operation - involving the recruitment of other men, pretending to be police officers - to abduct Mr Falconer.

Mr Falconer was handcuffed, his face was covered with a rag soaked in a chloroform-type substance, and he was placed inside a large unventilated metal box.

The box was transported to the Sydney home of Anthony Perish, who intended killing Mr Falconer. But Mr Falconer died on the journey which took more than two hours.

Then the box was taken to a rural property, where Mr Falconer's teeth were smashed to conceal his identity and his body cut up, before the packages were dumped in and near the Hastings River.

At last month's sentencing hearing, James Falconer told the judge his father died with "no dignity, no respect and no mercy".

His father had suffered worse than an "animal in an abattoir" and died "like a lion being pulled down by a pack of sniffing hyenas", James Falconer said.

Justice Price referred to the callousness of the way the body was disposed of, but said there was no suggestion that the dismemberment was done for any other reason than to hide the crime.

Anthony Perish told a psychologist that the grandparents' murders had "shattered the family's innocence and made him aware of how bad the world could be".

His brother referred to his great grief at their deaths and spoke of idolising his grandfather.

The judge set maximum terms of 24 years for Anthony Perish, 12 years for his brother Andrew, and 20 years for Lawton.