Investigators are still trying to work out the relationship between the pair.
Emergency services were called to a home in western Sydney late on Friday after receiving concerned reports about a resident's welfare.
Local police arrived to the Homebush West property about 11.30pm to find the body of the 65-year-old man in an on-site granny flat.
He has not yet been formally identified.
Another man, 31, was arrested at the scene and taken to Auburn police station.
By late Wednesday afternoon he was charged with murder.
He was refused bail and is set to appear at Parramatta Local Court on Monday.
On Saturday, the house remained cordoned off with police tape and forensic officers could be seen coming and going from the property.
Detectives from the NSW Homicide Squad are helping local officers with the investigation.
A knife was found but the cause of death has not yet been determined.
The two men were not related but police believe they knew each other.
NSW Police Superintendent Robert Toynton said investigators were seeking more information about the nature of the relationship between the pair.
"We have confirmed they know each other, but how they actually know each other will definitely be a line of inquiry that detectives are looking at today," he said.
Police were not pursuing other people of interest, Supt Toynton said.
"I want to reassure the community that this is an isolated, contained incident."
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