The bodies of up to five people, including children, were found in a Western Australia home by police on Sunday.
Investigators went to the house in suburban Perth after the man, aged in his 20s, presented himself to a regional police station.
Officers "discovered the deceased bodies of several people including children", police said in a statement.
Investigators did not confirm the number of victims, but said it was "believed to be up to five people". The ages and gender of those found were also not revealed.
"The man is currently in custody and assisting police with their inquiries. There are no ongoing concerns for public safety as a result of this incident," police added.
It was unclear how the victims died or any details about their connection with the man in custody.
Assistant Police Commissioner Paul Steel told reporters the investigation was still in a "very early stage".
The deaths came three months after a father shot dead his teenage son and daughter at their home in Sydney before turning the gun on himself.
It also follows the deaths of seven people, also in Western Australia, when a grandfather shot dead his wife, daughter and her four children in a mass murder-suicide in May.
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