The distraught father has just one question - why?

Mystery surrounds the circumstances of how Matthew Mitchell, 30, came to be lying by the side of a road in Sandy Beach on the NSW north coast last month, unconscious and with horrific head injuries.

Was he hit by a car? Bashed on a night out and left to die?

Detectives believe someone in the Sandy Beach community knows the answer to questions that have haunted the Mitchell family for the past month.

“Our son has died and all we want to know is why," Mr Mitchell said, shortly after his son died in the Royal North Shore Hospital on Thursday. Matthew had never regained consciousness.

“This has been an incredibly difficult experience for our family to go through and all we want is some answers, so please, if you know anything that could help, contact police," Mr Mitchell said.

What they do know is this: a passer-by found Matthew lying unconscious on the side of Graham Drive in Sandy Beach, about 20 kilometres north of Coffs Harbour, at 11pm on Saturday, August 17. He had serious head injuries.

Graham Drive intersects with the Pacific Highway, but is a rural road dotted with farmhouses.

Matthew was flown to the Royal North Shore Hospital with bleeding on the brain, and his family kept a vigil by his bedside until his death.

Mr Mitchell said he did not know if Matthew was bashed or hit by a car. It was torturous not knowing what had happened to his son, he said, and he would not wish the experience on his worst enemy.

"I'd swap places with Matthew if I could - but I can't and it's just horrible," he told the ABC earlier this week.

"I've never seen him in one fight, ever, I've never seen Matthew drunk even once.

"He's just a really loving, caring kid and he didn't deserve this."

Police from the Coffs-Clarence local area command established Strike Force Dartmoor to investigate the incident.

Acting Inspector Peter O'Reilly said detectives were yet to determine how Mr Mitchell came to be lying by the side of the road.

“While we have received some information from the community, the exact details of how Matthew came to be injured remain a mystery," he said.

“It is, however, clear that there are people in the local area who know how Matthew sustained his injuries.

“This is yet another tragic example of a young life lost - so we are again urging anyone with information to please contact us immediately."

Anyone who witnessed the incident, or has any information about Mr Mitchell's injuries, has been urged to contact Woolgoolga Police Station or Crime Stoppers on 1800 333 000.