Plea for help years after NSW hit and run

Four years to the day after the mysterious death of NSW man Matthew Mitchell, police and his family have launched a fresh appeal for information.

The 30-year-old was found with catastrophic head injuries on the side of a road in Sandy Beach, north of Coffs Harbour, late on August 17 in 2013.

He never regained consciousness and died in hospital after spending a month in a coma.

His injuries initially baffled investigators but in 2016 the deputy state coroner declared they were caused when Mr Mitchell was struck in a hit-and-run.

Detective Inspector Darren Jameson on Thursday said Mr Mitchell's family was still seeking answers and the investigation remained ongoing.

"Someone must know something so I urge you to pick up the phone and call us," he said in a statement.