All four people found dead in a suburban Melbourne home, including a 17-year-old, had synthetic opioids in their system.

Victoria Police on Thursday said their investigation into the Broadmeadows deaths in late June was ongoing, but confirmed the drug find.

“Preliminary tests have confirmed the presence of a synthetic opioid in the system of all four people,” the force said.

“No presence of fentanyl has been detected at this time.”

The deaths of the 37-year-old man, 32-year-old man, 17-year-old boy and 42-year-old woman are still being treated as non-suspicious.

The group was found dead in a Bicknell Court property early on June 26.

The Department of Health on Tuesday released a warning about cocaine in Melbourne being sold laced with synthetic opioid protonitazene, which is 100 times more potent than heroin.

The bodies were found by a relative and were all in the same area of the house, leading police to believe they were sitting together. There was no evidence to suggest there had been any violence.

Police believe one of the victims lived at the home and three others were visiting.

A neighbour, who was believed to be the uncle of the 17-year-old and was named as Corey by Nine News, raised the alarm.

He said he went to the home and knocked on the door but no one answered. Corey looked through a window and saw the teenager lying on the floor.

“I’ve come home to find family members in the house deceased and it just tortured me, shocked me,” he told Nine’s Today program.

“I think (it was) an overdose but I’m not too sure exactly.”

The 37-year-old man, 42-year-old woman and 17-year-old boy were known to police.

– with AAP