A farmer fatally shot an environmental officer in northern NSW two years ago because he was motivated by "personal hatred" and believed the man "had it in for him", a court has heard.

Key points: Ian Turnbull fatally shot environment officer Glen Turner in 2014

Ian Turnbull fatally shot environment officer Glen Turner in 2014 He pleaded not guilty to murder but entered plea of guilty to manslaughter, which was rejected

He pleaded not guilty to murder but entered plea of guilty to manslaughter, which was rejected Turnbull claims he was provoked after being harassed

Ian Turnbull, 81, Thursday pleaded not guilty in the NSW Supreme Court to murdering Glen Turner at Croppa Creek near Moree in July 2014.

Instead he pleaded guilty to manslaughter, but that plea was rejected by the Crown.

In his opening statement, Crown prosecutor Pat Barrett told the jury that on the afternoon of July 29, 2014, Turnbull drove up behind where Mr Turner, 51, and his colleague had parked on a dirt road and walked towards them carrying a rifle.

The court heard Turnbull was silent as he raised the gun, aimed it at Mr Turner and then shot him.

"That knocked Glen Turner to his knees, and he said 'Ian, what are you doing?''," Mr Barrett said.

The jury heard Mr Turner stumbled towards his vehicle and tried to hide behind it as another shot was fired.

Mr Barrett said Turnbull then pursued Mr Turner around the ute for more than 40 minutes before the victim attempted to run from the vehicle towards the tree line.

"He didn't get very far before the accused raised his rifle again and shot him," Mr Barrett said.

That shot, which struck Mr Turner in the back, was fatal.

The Crown told the jury the farmer was motivated by a personal hatred of Mr Turner.

"That was precipitated by a perception he had that the Office of Environment and Heritage, led by Glen Turner, had it in from him and the Turnbulls," Mr Barrett said.

The court heard Turnbull owned and had an interest in a number of properties around Croppa Creek and had been investigated by the Office of Environment and Heritage (OEH) for illegal land clearing in those areas.

The Crown told the jury that after Turnbull fired the first shot at Mr Turner he said, "You've ruined the Turnbulls... you're continually harassing us and taking us to court."

Victim's colleague pleaded with Turnbull

Mr Barrett said Mr Turner's colleague Robert Strange pleaded with Turnbull to let them go.

The jury heard Turnbull responded by saying, "the only way he's going is in a body bag".

The Crown said after Mr Turnbull fired the last, fatal shot, he turned to Mr Strange and said, "You can go now. I'll be at home waiting for the police."

He then got in his car, did a U-turn and drove away.

Defence says accused was 'a good bloke'

Defence counsel, Todd Alexis, started his opening statement by saying Turnbull was "a good bloke to know".

The court heard Turnbull had been married to his wife for 55 years and had four sons and 15 grandchildren.

Two of his children farmed in the local district.

Mr Alexis told the jury that while Turnbull shot Mr Turner, causing Mr Turner to die, it did not mean Turnbull was a murderer.

"The question arising is this: whether this was a planned encounter or one that happened by chance," he said.

He told the jury the court proceedings brought by the OEH had led Turnbull to have a "major depressive illness, which substantially impaired his moral judgment and capacity for self control".

The court heard Mr Turner had a lead role in the land-clearing investigation against Turnbull.

"The court proceedings brought significant pressure and worry to bear upon Mr Turnbull and his family," Mr Alexis said.

He told the jury that over the course of the trial they would need to consider whether Mr Turner harassed and intimidated Turnbull and if the farmer was worn down and provoked.

The trial is expected to last four weeks.