The family of a man who shot dead a NSW environment officer has been slapped with record fines totalling more than $700,000 for two illegal land-clearing offences in northern NSW.

In two decisions handed down on Tuesday by Brian Preston, Chief Judge of the Land and Environment Court, the son and grandson of the late farmer Ian Turnbull were ordered to pay a combined $708,750, plus the legal costs of the Office of Environment and Heritage (OEH).

Glen Turner, the OEH compliance official, was fatally shot by Ian Turnbull on July 2014 as he and a colleague were visiting the Croppa Creek area near Moree to investigate land-clearing by the Turnbulls.

Turnbull, 82, died of a heart attack in Long Bay prison in March this year, after being sentenced in June 2016 to 35 years in jail for Mr Turner's murder.