A man with injuries so 'horrific' that police thought he was brutally murdered was actually mauled to death by his pet dog, detectives say.

The 51-year-old man was found dead in his Nowra home, on the NSW South Coast, on Monday - with police initially treating his death as suspicious.

The man's neighbours on Douglas Street called police after becoming concerned for his welfare.

The man's neighbours on Douglas Street called police after becoming concerned for his welfare, before authorities found the 51-year-old's body inside his home in Nowra

Following an autopsy, investigators ruled out homicide as his cause of death.

Authorities now believe he had an epileptic fit before being mauled by his pet Staffordshire Bull Terrier.

'The injuries the victim suffered were horrific,' a homicide investigator told the Daily Telegraph.

'At the beginning there was really no way to establish an obvious cause of death.

'But now after further forensic tests at the scene and on the animal we are pretty confident that the dog for whatever reason attacked the man.'

A full report will now be prepared for the coroner.