Elderly farmer Ian Turnbull gunned down NSW environment officer Glen Turner "like a feral pig" in an act of revenge that has "broken" the dead man's family, his widow has told the NSW Supreme Court.

Mr Turner, a compliance officer working for the Office of Environment and Heritage, was shot dead by 81-year-old Turnbull during what should have been a routine departmental visit near Moree, in the state's north, two years ago.

The family of enviroment officer Glen Turner arrive for the sentencing hearing. Credit:Dean Lewins

Turnbull was found guilty of murder following a five-week trial, during which jurors heard the shooting followed years of tension and court battles over illegal clearing.

That dispute culminated on July 29, 2014, when Turnbull - armed with a rifle that he kept in his ute for shooting wild pigs and kangaroos - walked towards Mr Turner and his colleague Robert Strange at Croppa Creek, raised his weapon and wordlessly shot Mr Turner in the neck.