NSW farmer who shot man 'mentally ill'

The forensic psychiatrist treating an elderly NSW farmer who shot dead environment officer Glen Turner says his client suffers from major depression.

Ian Turnbull, 81, is on trial for murder after shooting Glen Turner multiple times at his Croppa Creek property near Moree in the state's north in July 2014.

Forensic psychiatrist Professor David Greenberg told the NSW Supreme Court on Thursday that his client was not thinking rationally when he shot the 51-year-old environment officer.

"In his perception, he saw the world in dark-tinted glasses," he said.

"He had a major depression, and it's not a temporary or transient thing."

The court has heard Turnbull drove to Talga Lane and, armed with a rifle, shot Mr Turner in the neck without saying a word.

The environment officer was involved in bringing illegal land clearing proceedings against Turnbull.

The crown has argued Turnbull's actions were premeditated, and that his anger towards Mr Turner was widely known by his family and friends.

Prof Greenberg rejected crown prosecutor Pat Barrett's claims, saying he wasn't satisfied the shooting was premeditated.

"He only developed the intention to shoot Mr Turnbull when he drove up to the laneway," he said.

Prof Greenberg said Turnbull did not try and present himself as mentally ill, but the brief of evidence he had collected on the farmer said otherwise.

He said Turnbull had a family history of mental illness and had been prescribed anti-depressants by his doctor after an episode in the 1970s.

His delusions on his situation were apparent, Prof Greenberg said.

"In his mind he perceived he was ruined ... he was bankrupt ... that everything was over," he said.

The trial continues before Justice Peter Johnson.