Nelson failed to comply with an industry standard that no trees be felled within two tree lengths of another worker.
He tried to check where Mr Ruri-Epapara was by radioing other workers and relied on their "vague" response, the judge said.
Mr Ruri-Epapara's mother, Marsella Edmonds, read her victim-impact statement, describing herself as "a broken mum". "My whole being aches to know my son would still be here if others had complied [with legislation]."
She said since the accident her family had spoken to more than 6000 people about health and safety.
"Robert's death will not be in vain. We are not victims, we are survivors."
Mr Ruri-Epapara's father, Robert Ruri, said the day his son died was "the darkest day of my life".
"I hope that Robert's death is not in vain and that serious lessons can be learned from this tragedy."
Judge McGuire said given Mr Ruri-Epapara didn't have a radio, Nelson should not have felled the tree unless he could see him.
"You thought you knew where this man was. You didn't," he said. "There can be no room for complacency any time, anywhere, any day."
Nelson is also a director and 40 per cent shareholder in Complete Logging Ltd, which in January was fined $60,000 and ordered to pay reparation of $75,000 in relation to the incident.
Judge McGuire also condemned the time it took for the case to get to court.
Speaking outside court, Mr Ruri-Epapara's stepfather Wiremu Edmonds agreed, saying the 18-month delay had led to uncertainty and speculation.
He said his family felt for Nelson, who had been friends with Mr Ruri-Epapara.
"He knows he's made a mistake, we know he's made a mistake."
The two families had been pushed apart but he hoped time would heal and bring understanding, he said.