Turnbull’s barrister Todd Alexis SC closed the defence case on Wednesday by highlighting expert evidence that his client had been suffering “an extreme form of depression” when he shot Glen Turner with a rifle at his property, north of Moree, in July 2014.

“Should he be condemned as a murderer when the psychiatric evidence ... shows that his capacity was substantially compromised by a major depression? No, he shouldn’t be, you may think,” Mr Alexis told the NSW Supreme Court jury.

Turnbull has been on trial for nearly five weeks after pleading not guilty to murder.

The court has heard the farmer opened fire on the 51-year-old Office of Environment and Heritage compliance officer with a rifle he used for shooting vermin, after years of tension over claims of illegal land-clearing on the Turnbull properties in the Moree area.

Mr Alexis said the case was “bizarre” in the sense that his client’s actions were “completely at odds” with the way he had previously led his life.

“You might think that the significantly out-of-character conduct rather strongly underlines the severity of the mental illness at the time,” he said.

Depression affected people in different ways and sometimes went unnoticed, he said.

“Sometimes it can lead to tragic consequences, and this case is a regrettable example,” Mr Alexis said.

Turnbull shouldn’t be condemned for not recognising his illness or getting treatment, and his “moral blameworthiness” for Mr Turner’s death should be reduced to the lesser charge of manslaughter, Mr Alexis told the jury.

Justice Peter Johnson was due to begin his summary of the case on Wednesday afternoon.