A farmer who gunned down a NSW environment officer during a protracted and terrifying ordeal has been jailed for at least 24 years.

Ian Turnbull, now 81, used a hunting rifle to murder Glen Turner, 51, who was on public land with a colleague on July 29, 2014, near the farmer's property at Croppa Creek in the state's north.

In the NSW Supreme Court on Thursday, Justice Peter Johnson jailed Turnbull for a maximum of 35 years for the "terrifying and shattering" murder and for detaining his colleague for advantage.

Murderer Ian Turnbull is escorted out of court during the trial. Photo: James Alcock

Justice Johnson said it was a de facto life sentence for the farmer, who has a life expectancy of just eight years.

Mr Turner's wife, Alison McKenzie, as well as his family, friends and Office of Environment and Heritage colleagues watched as the sentence was handed down.

During a judgment spanning an hour-and-a-half in front of a packed courtroom, Justice Johnson said he did not accept Turnbull had shown genuine remorse for the murder, or detaining Mr Turner's colleague Robert Strange at gunpoint for more than 20 minutes. 

Glen Turner, his wife Alison McKenzie and their children Jack and Alexandra. Photo: Tracy Fulford Photography

Turnbull showed no emotion as the sentence was read out and only nodded once and waved at his wife, Robeena, seated in the front of the court. 

The sentence means Turnbull would not be eligible for parole until 2038, but Justice Johnson accepted he would die in prison before then.

Turnbull was found guilty of murder late last month following a five-week trial where the jury heard the shooting followed years of tension over illegal land-clearing.

Glen Turner's widow Alison McKenzie, left, and his sister Fran Pearce. Photo: Emma Partridge

Turnbull pleaded not guilty to murder but guilty to manslaughter on the grounds of substantial impairment due to mental illness.

However, the jury rejected Turnbull's defence, after one day of deliberations.

The defence had maintained Turnbull's major depressive order was caused by years of tension over illegal land-clearing on the family's properties at Croppa Creek.

Pictures of burning stacks of native vegetation taken by Office of Environment and Heritage officer Robert Strange in Croppa Creek just before the killing of Glen Turner. Photo: NSW Police

A farmhand alerted Turnbull that Mr Turner was near his family properties, before the farmer drove to Talga Lane, Croppa Creek, in the early evening of July 29, 2014.

Turnbull confronted Mr Turner and shot him in the neck.

He then chased him around a car for at least 22 minutes and fired a number of shots before shooting him in the back - the shot that proved fatal.

Mr Strange told the trial he had pleaded with Turnbull to put the gun down.

Afterwards, Turnbull got in his car and drove home, where he was arrested.

During the trial, Turnbull gave evidence that he intended to kill Mr Turner when he hunted him down near his property and chased him around the car.

He has been in custody since the night of the murder.

Northern Daily Leader, with AAP