Police did not confirm the nature of the mother’s injuries but Australian media reported she had been beheaded during the incident in the Australian city of Sydney on Saturday night.
Detective Superintendent Brett McFadden described the attack as “up there with one of the most significant, most horrific scenes police have to deal with”, adding that the injuries to the victim “were extensive”.
He said police understood there had been an argument between the woman and her mother, named in Australian media as Rita Camilleri, 57, that had become violent.
Neighbours called police and the mother’s body was found in the home, with multiple injuries.
The daughter was arrested outside the home of a neighbour and the four-year-old boy was taken to hospital with what police said was a minor head injury.
He has since been released.
Police officers who investigated the scene had been offered counselling.
Det Supt McFadden added: “The detectives are now working meticulously through the information available to us, but it’s a very difficult and challenging investigation.
“It is also in its very early stages and as you’d understand there is still a long way to go in this inquiry.”
The Australian Broadcasting Corporation quoted him as saying: “We understand at this stage that a number of knives were used during the course of the incident.
“I am not able to specify what kind of knives they were at this stage.”
The daughter, named in Australian media as Jessica Camilleri, 25, has appeared in court and been referred for a medical assessment.
She told the court that she suffered from mental illness and other medical conditions, adding: “I couldn’t even wash myself properly when I had to shower to get all the blood off”.
