The stabbings occurred at the Thomas Embling Hospital in Fairfield, in Melbourne's inner-north about 11pm (AEDT) last night.

Detective Sergeant Wayne Cheesman (Cheesman) said a 49-year-old patient armed himself with a large kitchen knife and fatally stabbed his 54-year-old roommate several times in the upper body.

He said the man then went into the room next door and stabbed a 36-year-old man who was in bed.

Det Sgt Cheesman said the motive for the attacks was not yet known.

"I know the roommate and the offender were considered quite friendly," he told reporters at the facility.

"They would often go out on day pass and leave and go fishing together, and what brought on the altercation, we don't know."

Det Sgt Cheesman said he could not yet release the names of the three men, but all had committed murder.

"It's my understanding that both the victims and offender were in the facility after committing the offence of murder," he said.

Police were also investigating where the man got the knife from.

"Within the facility there are self-contained living quarters. So there's kitchen and cooking facilities," he said.

"So, there are knives in there and whether the knife he had or whether it's a knife he brought in, we're still trying to ascertain where it came from.'

Det Sgt Cheesman said the 49-year-old man was being assessed at the Fitzroy Police Station for whether he was fit to be interviewed.

He is expected to be charged and appear at the Melbourne Magistrates' Court on Thursday.

He said none of the other patients witnessed the incidents, but it was believed a staff member saw the second stabbing.

Police have interviewed 14 patients and staff.

"Some of the patients are obviously upset but quite calm," Det Sgt Cheesman said.

"The staff are genuinely quite upset because they work closely with the patients."

The 118-bed hospital is operated by the Victorian Institute of Forensic Mental Health (Forensicare), a section of the Department of Human Services.

The facility is secure and caters mainly for patients from the criminal justice system who need psychiatric assessment, care or treatment.

A security upgrade, including new access controls and the re-organisation of the communications room, was completed in January this year.