Police are investigating the mysterious circumstances surrounding the death of a 21-year-old man who was found badly injured and unconscious on the NSW Central Coast.

Jayden Mason was lying on the road with severe head injuries when a passing motorist spotted him in Lake Haven about 10.30pm on Saturday.

He was treated at the scene by paramedics and taken to John Hunter Hospital.

On Sunday, his family made the heart-rending decision to switch off his life support after they discovered the extent of his brain damage.

Police say they are investigating 'all lines of inquiry' into what caused the death, including whether Jayden was bashed or hit by a car.

The young man's family is struggling to come to terms with the tragic death.

Jayden's friends paid tribute to him as 'one of the greatest people' they knew.

'I love you forever Maso, thank you for all the memories growing up,' a friend wrote on Facebook.

'You will never be forgotten. You will live forever in our hearts.'

Police are appealing for information, and would like to speak with a woman who stopped to help the injured 21-year-old.

She is described as being in her 50s, and was driving a dark blue Ford Falcon sedan towing a box trail.

AAP