Homicide Squad Detective Inspector Dean Thomas said the bodies of a 32-year-old man, 37-year-old man, 42-year-old woman and 17-year-old boy were discovered in the lounge room of the home in Broadmeadows.
A neighbour and family member of the deceased teenage boy called emergency services just before 2am.
Cory Lewis told reporters he peered inside to spot his nephew’s body on the floor, before smashing a window to gain access.
“I’ve come home to find family members in a house, deceased,” he said.
“I found them and it’s tortured me.”
Homicide detectives have established a crime scene but are not treating the deaths as suspicious.
Thomas said police had discovered some items in the house that “may be drug paraphernalia”.
He said detectives were “keeping an open mind”, saying the cause of death could be anything.
“We don’t know what has caused their death,” he said.
“We’ve got to keep an open mind to this.
“It’s unusual that we find four people deceased in the one property, potentially from a drug overdose but we’ve got to keep an open mind until we can confirm that’s the case, it could be anything.
“The families will want answers as to what has occurred.”
Thomas said three of the four people, including the teenage boy were known to police.
A report will be prepared for the coroner.
The investigation is ongoing.
Anybody with information is urged to contact Crime Stoppers on 1800 333 000.