Police arrived at the north-eastern suburbs property about noon WA time (2pm AEST), where they found the bodies of up to five people - including women and children.
The age and gender of those found has not yet been revealed, although it is thought "there was at least one woman".
Forensic police are now investigating the scene.
"The man in custody is the person who attended the regional West Australian police location".
WA Police Assistant Commissioner Paul Steel said a man in his 20s presented to a regional police station today.
He did not confirm the exact number of dead, nor did he detail what the relationship was between the man in custody and the deceased. The Assistant Commissioner assured the community there were no on-going concerns for public safety as a result of this incident.
"It is something you never would think would happen in this area", she said.
"Specialist forensic police will investigate the scene".
"It's horrid to think that in such a lovely area, that's calm and so quiet, that something so tragic has taken place", she said.
In a statement via 7 News Melbourne, Metropolitan Region Assistant Commissioner Paul Steel confirmed the "tragic event".
In May four children and three adults were found dead in a murder-suicide near Margaret River in the state's south-west.
Steel stressed that this was very early in the investigation so he could not provide many details.
"We're all in complete shock".
Forensic police and the homicide squad were undertaking a "meticulous" investigation of the Bedford crime scene which "could take a number of days".
Ms Manfredini said she did not know the occupants of the home personally, but would often wave and say hello as she walked past.
A mother and her two young children were murdered in July in the Perth suburb of Ellenbrook.
"It does send a ripple through the community of Western Australia".
Peter Miles, 61, his 58-year-old wife Cynda, their daughter Katrina, 35, and her four children - daughter Taye, 13, and sons Rylan, 12, Arye, 10 and Kadyn, eight - were found dead at a farm in Osmington, in Australia's worst mass shooting since the Port Arthur massacre in 1996.