First Constable Tyson Lopez fired the shots at Carly Pirronelli, 26, on March 8, 2024, after she ignored orders to drop her knife.
"We needed to stop her," the officer told a Victorian inquest into her death.
"I had no time. We had to act then."
First Const Lopez and Senior Constable Michael Zappiello arrived at the home in Melbourne's northeast about 6.40pm after a neighbour raised the alarm.
The two officers found a large pool of blood on the back patio and Carly leaning over her mother Joanne Perry, 53.
"Crudely, I believe she was trying to decapitate or cut the head off," Sen Const Zappiello told the inquest on Tuesday.
The two officers pointed their firearms at Carly and repeatedly ordered her to drop the knife.
She briefly did so before saying "it's my mum", and resuming her attack.
Both officers came to the conclusion the only way to stop Carly would be to fire a shot, the court was told.
Sen Const Zappiello yelled out "fire fire", before First Const Lopez set off three bullets at Carly.
The senior constable told the court he expected Carly to stop her attack so he could arrest her and provide first aid to Joanne.
But instead, Carly continued to cut at her mother's neck as if nothing had happened.
"It was like someone playing a violin," Sen Const Zappiello told the court.
"It was back and forth and it wasn't stopping."
First Const Lopez also did not see any change in Carly's demeanour so he fired a further five shots.
"It wasn't until the sixth or seventh shot I fired that I was starting to see some change in behaviour," he told the court.
"After the seventh, she started to slouch over Joanne."
Sen Const Zappiello said he moved Carly into a recovery position but felt "in over his head" to perform CPR given her extensive gunshot wounds.
The officers also believed Joanne was dead from her injuries.
Paramedics arrived a short time later and confirmed both women were dead.
Sen Const Zappiello told the court Carly had appeared either drug-affected or under a psychiatric episode at the time of her death.
"She wasn't responding like a person in a normal state of mind," he said.
First Const Lopez said while he was reluctant to fire the shots, he did not believe he could have done anything differently.
The circumstances leading up to the deaths of Carly and Joanne - as their family would prefer they be known - is a key focus of the inquest.
Carly's declining mental health in the months leading up to the incident is also being investigated in the hearing before Deputy State Coroner Paresa Spanos.
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