Gabriel Messo, who had diagnosed bipolar, was shot in the chest three times by Constable Emmanuel Andrew at John Coutts Reserve in Melbourne's north on the afternoon of July 16, 2020.
The 30-year-old was in the middle of a violent 17-minute assault against his mother Lilla Messo when Constable Andrew and First Constable Rebecca Churcher arrived at the scene.
Giving evidence at an inquest on Thursday, the now Senior Constable Churcher recalled her colleague telling the man repeatedly to stop his attack.
Mr Messo said something to Const Andrew before going back to stabbing the woman with a stick, she said, and her colleague pointed his firearm at the offender while still telling him to stop.
She also got her firearm out of her holster, telling State Coroner John Cain: "I naturally felt that this was the appropriate action".
When Mr Messo didn't stop stabbing, her colleague shot the man once and it was like the attacker "didn't even flinch", Sen Const Churcher said.
Const Andrew shot him again and when Mr Messo advanced on the officers he fired a third time, she said.
Both Sen Const Chucher and Const Andrew didn't have Tasers on them.
It wouldn't have been appropriate to use a baton on Mr Messo even after he dropped the stick, given they would have had to get close enough to also be stabbed potentially by another weapon, she said.
Likewise, it wouldn't have been appropriate to use capsicum spray, Sen Const Churcher said, because they were outside and open to the elements.
She saw the colour drain from Mr Messo's face and fall back, and she went straight to his mother to find her face swollen, covered in blood and deep open wounds.
Talking through tears, Sen Const Churcher said the woman's mouth was filled in mud and she tried to scoop it out.
She told Const Andrew to start compressions on Mr Messo and, when her colleague said "shit, I didn't have my camera on", she assured him hers was on.
Sen Const Churcher looked down and saw it was green but not active, she told the coroner. She hit the camera again to turn it on.
Mrs Messo was taken to hospital with life-threatening injuries after she went into cardiac arrest four times.
She survived the attack but now lives with an acquired brain injury.
The inquest is looking at the circumstances surrounding Mr Messo's death - his mental health treatment, his arrest on July 15, and the actions of Victoria Police.