Palmerston North police are likening their investigation into how a man received severe head injuries to a jigsaw puzzle.

The 34-year-old man is in the Neurological Unit at Wellington Hospital, after being found in the backyard of a Palmerston North house where a party was being held. He was unconscious and had a fractured skull.

Police and an ambulance were called when a security guard arrived to deal with a noise complaint.

Detective Sergeant Dave Thompson says people at the party claim they found the man unconscious on the street and carried him to the backyard where they were giving him First Aid. He says the partygoers say that is when the security guard arrived and called an ambulance and police.

Detective Sergeant Thompson says there are still a lot of unanswered questions, and it is not yet clear if a crime has been committed.