A MAREEBA stockman who killed his 88-year-old boss with an axe and gun two years ago was anxious and stressed in the lead-up to the incident due to increasing demands from the elderly man, a court has heard.

Robert John Wilson, 61, has pleaded not guilty to murdering Mt Carbine grazier David Thornton on March 12, 2010, butthhas admitted he hit the manthtwice in the head with the blunt end of an axe before shooting him.

On day three of a Cairns Supreme Court trial yesterday, two psychiatrists said he was most likely impaired at the time of the killing due to his bi-polar disorder.

The court has heard Mr Wilson's role at the sprawling Curraghmore Station went from stockman to carer as Mr Thornton aged. According to psychiatrist Dr Donald Grant, he was not coping with the situation.

"He was anxious and stressed by the demands from Mr Thornton,'' Dr Grant, who assessed Mr Wilson after the incident, told the court.

Dr Grant said the accused was probably experiencing a "mild depressive phase'' of his illness because he was not taking his medication at the time.

"I think the capacity of control might have ... been significantly impaired,'' he said.

The court has previously heard Mr Wilson's wife was so worried about his condition she had secretly planned a move to Darwin upon his return to Mareeba, but he was forced to stay at the property longer than planned because no one could relieve his post.

Dr Barbara McGuire also told the court she thought Mr Wilson was impaired at the time of the incident.

"I think had he not had a mental illness, he would have dealt with this differently,'' she said.

"I think he does represent someone with diminished responsibility.''

The trial continues before Justice Jim Henry today.

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