Homicide detectives went to the home on Coode Street in Bedford yesterday after a man in his 20s handing himself into the regional police station.
'The man currently assisting police remains in custody, no charges have been laid at this time,' police said in a statement.
WA Police Assistant Commissioner Paul Steel said, 'This is a tragic event and it will no doubt have an impact not only on the family and friends of the deceased but for the whole of the community and those first responders who are faced with attending a scene of multiple deceased people.'
Steel added that he could add no further detail at the current time.
'Depending on the size of that crime scene that will take a period of time, and it could be a number of days,' he said.
Homicide detectives and specialist forensic police will continue to comb the Coode Street home for evidence and clues to what happened after the shocking discovery yesterday.
It's believed the man was known to the victims, although police have not confirmed this.