Police found the remains of the toddlers, as well as those of their mother and grandmother at an unassuming detached family home near the south-western city of Perth.
The discovery came after a man, aged in his 20s, visited a regional police station earlier Sunday, investigators said.
Western Australia police said Monday the “five people killed were twin two-year-old girls, a three and a half-year-old girl, their mother and grandmother”.
“The man currently assisting police remains in custody, no charges have been laid at this time.”
No further details about when and how they were killed, or the family’s relationship with the man, were released.
A neighbour told national broadcaster ABC he had returned from a holiday to “silence in the street”.
“We had some friends staying here who have also mentioned that they didn’t see or hear anybody next door for the week that we were away… we could hear and see the kids playing in the backyard quite often.”
Other neighbours spoke of a community that was close, calm and peaceful.
Mass killings are rare in Australia, but this appears to be the third such family tragedy to hit Western Australia state in recent months.
In July a man allegedly killed his mother and two siblings. A grandfather shot dead his wife, daughter and her four children in murder-suicide in May. —AFP