A New South Wales farmer who shot and killed an environment officer involved in land clearing prosecutions against his family has been found guilty of murder.

Ian Turnbull drove out in search of Glen Turner, after learning that he was in the Croppa Creek area near Moree in July 2014.

During his trial, Turnbull admitted taking his rifle and firing several shots at Mr Turner.

The question the jury was told to answer was whether his actions amounted to murder or manslaughter.

Members of Mr Turner's family, including his partner Alison McKenzie and sister Fran Pearce, embraced one another in court after the verdict was read out.

Turnbull showed little emotion, but appeared to shake his head slightly as the jury left the courtroom.

He had told the jury earlier in the trial that calmness came over him after he fired the first of three shots to hit Mr Turner, over a period of at least 20 minutes, at the side of a road.

It happened in front of fellow environment officer Robert Strange, who pleaded with the 81-year old to let them go, so he could get help for his seriously wounded colleague.

Turnbull refused, telling Mr Strange the only way Mr Turner was leaving was in a body bag.

After firing his final and fatal shot, Turnbull said he was going home and the police would know where to find him.

Defence argued for manslaughter conviction due to depression

Prosecutor Pat Barrett told the jury during his submissions that the attack "was deliberate and considered and bares upon his state of mind at the time".

NSW environment officer Glen Turner was fatally shot on a property north of Moree in July 2014. ( Supplied by the Turner family )

The defence argued Turnbull should only convicted of manslaughter, because he was substantially impaired by a major depressive illness.

Turnbull's barrister Todd Alexis SC said the ongoing litigation involving the Office of Environment and Heritage over land clearing, had created a "sea of paper".

He said his client thought he was being persecuted and had become obsessive about Mr Turner, even though he had barely seen him during the two years before the shooting.

"It was almost a fantasy in his mind," Mr Alexis said.

"He became fixated with it."

The jury started deliberating just after 2:00pm on Thursday afternoon, after a four-week trial.

Mr Strange was also in court to hear the guilty verdict over the murder of his colleague.

'The murderer was portrayed as the victim'

Mr Turner's widow Alison McKenzie said the verdict brought "sheer relief".

"We're never going to be able to fill the void that's been left in our lives but we got the right result," she said outside the court.

Glen Turner's widow Alison McKenzie (left) and his sister Fran Pearce after the verdict. ( ABC News: Nick Dole )

She said her partner's character had been "decimated" by Turnbull's defence team.

"This whole trial has turned into a platform for the Turnbull's hatred of the native vegetation laws," she said.

Mr Turner was remembered by his sister as "a man who was full of vitality and he had a passion for life".

"The murderer was portrayed as the victim — a poor depressed respectable farmer, driven to despair by the Office of Environment and Heritage," Fran Pearce said.

"In reality, he is a wealthy property developer who simply refused to accept the law applied to him.

"Today's verdict does not bring Glen back, but we do take some comfort knowing that justice has been done."