Wheat farmer Ian Turnbull ambushed Glen Turner on a public road and opened fire while the environment compliance officer was inspecting an unrelated site, police will allege.

Mr Turner was not on the Turnbull family's properties and was not serving a notice for illegal land clearing when he was allegedly shot several times, throwing serious doubt on claims by Mr Turnbull's family that the 79-year-old farmer had been pushed to the edge by repeated harassment.

Crime scene: Police block off the road at Talga Lane on the Newell Highway at Croppa Creek last week. Credit:Cady Anderson, Moree Champion

Detective Inspector Gavin Rattenbury said police will allege Mr Turner and a colleague from the Office of Environment and Heritage (OEH) were carrying out unrelated duties on the side of Talga Lane, near Moree, last Tuesday evening when Mr Turnbull allegedly pulled up beside them, armed himself with a rifle and opened fire.

A bullet in the back killed Mr Turner but it is understood that more than one shot hit the 51-year-old father-of-two. Mr Turnbull then drove home, where he was arrested four hours later.