NSW farmer 'calm' during fatal shooting

Elderly farmer Ian Turnbull has described to a jury feeling calmness come over him as he shot an environmental officer dead on his northern NSW property.

After fatally wounding Glen Turner at Croppa Creek in July 2014, Turnbull turned to the man's colleague and said: "You can do what you like now, I'm going home... people know where to come and get me".

The 81-year-old entered the NSW Supreme Court witness box for the first time at his murder trial on Monday.

He has admitted the shooting but pleaded not guilty to murder, with his lawyers arguing he'd snapped under the pressure of persistent legal action.

Turnbull told the jury he'd been extremely nervous, but after firing the first shot at Mr Turner, a calmness came over him.

Asked why he didn't call police or an ambulance for Mr Turner, Turnbull said he assumed the man's colleague would do it.

"I wasn't in a state to think like that, rationally think," he told the court.

The trial continues on Tuesday.