Four people are fighting for their lives in intensive care and five others are in hospital after a suspected mass drug overdose at a property housing foreign nationals - believed to be backpackers or students - in the inner Perth suburb of Victoria Park.
Emergency services were called to the Colombo Street home just before midnight after reports nine people were unconscious or violently ill.
Seven men and two women were rushed to three separate hospitals around the city.
Three men were taken to Royal Perth Hospital; two of them are in intensive care.
Two men and a woman were taken to Sir Charles Gairdner Hospital, where two of them are also in intensive care.
Another two men and a woman are in Fiona Stanley Hospital, but their conditions are not known at this stage.
Two students, from Germany and Colombia, have told the ABC they were not involved in the incident, but were shocked to find out what had happened when they returned to the house last night.
Police have cordoned off the property and say they hope to release further information about the incident later today.
It is believed some substances have been removed from the property for analysis.
Organised Crime has been notified due to the presence of drugs, police said.
Neighbour Garry Baden-Davidson said it was common to see different people staying at the property.
"It's like a backpackers' [hostel], they come and go. For two weeks, one lot will come and then another lot will go out," he said.
"This lot here … the only noise I heard from them was on New Year's Eve and it wasn't really that loud either.
"It's pretty sad really, young people like that to get into that situation, really sad, they seem like happy-go-lucky type people, they come and go, they all seem to have part-time jobs and work."