A woman has allegedly decapitated her mother in front of the 57-year-old's four-year-old grandson in their western Sydney home.
The dead woman's 25-year-old daughter was arrested in the front yard of a neighbour's home, charged with murder and will appear in Parramatta bail court later today.
"This is up there with one of the most significant, most horrific scenes police have had to face," Nepean Police Area Commander Detective Superintendent Inspector Brett McFadden told reporters.
He would not confirm the decapitation reports, but said a number of knives were used in the "horrific" attack on the older woman in St Clair on last night.
"There is no stepping away from the fact that this was horrific and a significant attack, and the injuries that the victim suffered were extensive," Det McFadden said.
The four-year-old boy, who was at home at the time, suffered a minor head injury and was treated at the Westmead Children's Hospital.
Police say the child is the nephew of the accused woman.
"He is being cared for by his family and getting medical support during the course of the day. The family is distraught as to what has taken place," Det McFadden said.
The family was known to police but what unfolded last night was "not foreseeable", he said.
Strike force Comeroy has been set up to investigate the incident.
"It is a very difficult, challenging, confronting investigation," he said.
"A lot of considerations need to be made, such as the background about what has happened at the address."
Neighbours called police around 11.40pm with "quite specific" information about the violent argument at the St Clair Avenue home where both women lived.
Knives have been seized for forensic examination and several crime scenes have been established inside the home.
Support is being provided to the first responders who were confronted with the distressing scene, police said.
"It is something that while police are trained to respond to all manner of incidents, it just took the extra level of professionalism and tenacity on their part to actually achieve the results they did last night," Det McFadden said.