The Moree farmer convicted of shooting and killing a New South Wales environment officer has died of a heart attack in Sydney's Long Bay prison.

Ian Turnbull, 82, was sentenced to 35 years in prison after he was found guilty of murdering Glen Turner over a land dispute at Croppa Creek in July 2014.

A spokeswoman for Corrective Services NSW confirmed Turnbull died in the Long Bay prison hospital yesterday.

NSW environment officer Glen Turner was shot dead on a property near Moree. ( Supplied by the Turner family )

Turnbull stood trial for the murder of Glen Turner in June 2016.

The trial heard Turnbull was facing prosecutions by the Office of Environment and Heritage (OEH) in the Land and Environment Court over illegal land clearing, which he had continued to do after officially being told to stop.

The court also heard Turnbull believed he was being singled out by State Government officials.

In sentencing, Justice Peter Johnson said Turnbull intended to kill Mr Turner and that his sentence ought to reflect that.

The judge also said it was a de facto life sentence due to Turnbull's age.

"I have taken into account that this sentence will almost certainly constitute a de facto life sentence with the offender dying in custody before the expiration of the non-parole period," the judge said.

"I am satisfied that no lesser sentence is appropriate in all the circumstances of the case."

Turnbull was given a non-parole period of 24 years in prison.

He had served less than a year of the sentence when he died yesterday.

Turnbull 'died a murderer'

Glen Turner's friend Graeme McKenzie said Turnbull's death meant there would not be an appeal, which might have been traumatic for Mr Turner's family.

"It's good now that the appeal is no longer required because it was just going to be another burden on [his widow] Alison and Glen's family to endure, so, in that regard, I'm happy for them," Mr McKenzie said.

"He has died in prison as a murderer, so I think that's probably the bottom line as far as that's concerned.

"You can't replace Glen and it's a huge loss for the remainder of Glen's family, Alison and the kids, but at least this puts a full stop as far as those legal proceedings."

Recently, the family of Mr Turner spoke out ahead of a new documentary to be released on the circumstances surrounding his death.

His widow, Alison McKenzie, said there was misdirected sympathy for Turnbull.

She said her late husband was just doing his job.