Village Voice News - National - Police unsure who stabbed mum and kids

A father raised the alarm after his wife was fatally stabbed and his two daughters injured in Sydney's north, police say.

Police were called to the house in Cambewarra Crescent at Berowra Heights at 5am (AEST) on Thursday, after a call from a neighbour.

They found a 29-year-old woman dead with a number of stab wounds to her upper body and a four-year-old girl and her five-year-old sister injured.

Police later said the younger girl had suffered serious neck injuries.

Her older sister remains in a stable condition in the children's ward of Hornsby Hospital, a hospital spokeswoman said.

A 31-year-old man and a nine-year-old boy at the house were not hurt.

Superintendent Craig Rae said the children's 31-year-old father had gone to the neighbour to raise the alarm after the incident.

Supt Rae said it was too early to speculate on what had happened.

He told reporters inquiries so far had not ruled the father in or out, or the possibility of an intruder.

When asked to comment on earlier speculation the mother had injured the children, Supt Rae said: "It's certainly part of our inquiry."

"We'll be looking at all avenues of inquiry and keeping an open mind at this stage of the investigation," he told reporters at the scene, which was cordoned off on Thursday morning with police vehicles and tape.

Supt Rae said he could not confirm whether the five people in the house were awake or asleep at the time of the incident or where the body of the woman was found.

"Because we are at such early stages, my feelings at the moment are, obviously, for the three young children who have unfortunately lost their mother," he said.

"We're obviously trying to speak to family members and to provide support and counselling for them."

Police officers were being debriefed after attending the scene, he said.

"It's a very difficult situation for all the police," he said.

Supt Rae said it was too early to give any information on the weapon or whether anything had been taken from the house.

The five people in the house were all believed to be related and the family were the only ones in the property at the time.

The four-year-old girl has been taken to The Children's Hospital at Westmead with serious injuries.

The five-year-old girl has been taken to Hornsby Hospital with minor injuries.

The nine-year-old boy is with his father and police.

A neighbour, Michelle, said there was nothing remarkable about the family.

"It's pretty sad, isn't it?" she said.

"It's just in a quiet suburb.

"Everybody just lives their own lives, really."

A family friend says the woman who was stabbed to death was a "lovely girl".

The man, who asked not to be named, said he knew the woman and her 31-year-old partner and the three children.

The dead woman was his son's first girlfriend, the man said, adding that he had also coached her partner in soccer.

"She's a beautiful girl ... lovely girl, terrific kids," he said at the scene.

He said the couple had been together for a number of years and as far as he knew, the relationship was fine.

"I've got no idea (what happened)."

He said the house was owned by the woman's mother, who had been away.

"I think they were just minding the house," he said.

His family had been hit hard by the news, he said.

"Our kids grew up together, they were all upset.

"I think you can tell my reaction - shock horror."