Ian Turnbull, who had been serving a maximum 35-year jail term at Long Bay Correctional Complex for the shooting murder of 51-year-old Glen Turner, died in Prince of Wales Hospital in Randwick on Monday.

A Corrective Services spokeswoman confirmed the inmate died after being taken to hospital by ambulance on March 20. He was suffering a terminal illness, the spokeswoman said.

He is also understood to have suffered a heart attack while in custody.

Last year, Turnbull was sentenced to a maximum of 35 years in prison, with a non-parole period of 24 years, for the "terrifying and shattering" murder of Mr Turner at Croppa Creek, near Moree in north-western NSW, in July, 2014.

Mr Turner's widow, Alison McKenzie, said Turnbull's death meant her family would not have to endure the torment of an appeal. Last month, Turnbull had lodged an appeal of his sentence that was due to be heard on May 31.

"He died a convicted murderer and spared us from the heartache of an appeal in court," Ms McKenzie said in a statement.

"He was able to receive medical attention at his time of need, something he denied his victim, my husband Glen."

Mr Turner, an officer for the Office of Environment and Heritage, had been investigating Turnbull for illegal clearing of native vegetation, and was carrying out what should have been a routine departmental visit with a colleague, Robert Strange, when he was murdered.

Mr Turner and Mr Strange were standing on public land near Turnbull's property at Croppa Creek when Turnbull used a hunting rifle that he kept in his ute for shooting wild pigs and kangaroos to kill Mr Turner.

His trial heard that, on the day of the shooting, Turnbull walked towards Mr Turner and Mr Strange, raised his weapon and wordlessly shot Mr Turner in the neck.

Turnbull then chased him around a car for at least 22 minutes and fired a number of shots before shooting Mr Turner in the back - the shot which proved fatal. Turnbull also detained Mr Strange at gunpoint.

The shooting followed years of tension and court battles over illegal clearing. Turnbull was facing prosecutions by the Office of Environment and Heritage in the Land and Environment Court over illegal land clearing, which he continued to do after officially being told to stop.

When sentencing Turnbull last year, Justice Peter Johnson told the court that the 35-year maximum sentence was a de facto life sentence for the farmer.

"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."

He also said he did not accept that Turnbull had shown genuine remorse for the murder.

"The events ... involved the offender prolonging the process of murdering Mr Turner, thereby heightening the terror to which Mr Turner was subjected, before the final and fatal shot was fired," he said.

During a sentencing hearing in Sydney last year, Ms McKenzie said she could not comprehend how someone could hurt another person in that manner.

"He [her husband] was treated like a feral pig," she said in a statement read to the court by a family member.

She said her family was "broken" by her husband's death.

"The world had swallowed me up," she said of the moment when she found out he had died.

With AAP