Three years ago, Courtney Topic was shot dead by police while brandishing a knife outside a fast-food outlet in Sydney's western suburbs. She was 22.

That day the family's world fell apart.

"We want everyone to know the beautiful girl Courtney was," her mother Leesa Topic told 7.30.

"We are heartbroken. This is killing us."

Ahead of an inquest into their daughter's death, Leesa and Ron Topic are speaking publicly for the first time, asking if things could have been handled differently.

"Never, ever was Courtney violent, she wouldn't hurt a fly," Ms Topic said.

They are questioning the police response that day, and want people to know about the kind, gentle girl who died in such a violent way.

Statistics revealed on 7.30 last week showed that more than half of the people shot dead by NSW Police in the past 20 years had a mental illness.

It is a vexing issue raising questions about the level of police training and whether it equips them to deal with these extremely difficult, life or death, situations that can leave both families and police devastated.

"It's so far removed from who Courtney was, so far removed from everything she represents, everything she was brought up with, everything she knows," Ms Topic said.

"She is a beautiful person who wouldn't hurt anybody and, for whatever reason, something went the worst kind of wrong that day, went horrifically wrong.

Courtney's father Ron Topic added: "What happened that day should never have happened."

'A blank canvas … that's how I see people's expressions'

Leesa Topic says Courtney sometimes struggled in social situations. ( Supplied: Leesa Topic )

Courtney Topic grew up with her three brothers in Sydney's western suburbs.

Soon after graduating from high school, she was diagnosed with Asperger's syndrome, a form of autism.

Her father asked her to explain how it affected her.

"She said, 'Dad, you know how I like art? Do you know what a blank canvas is? That's how I see people's expressions. I don't know if people are happy or sad or if they want to engage or they don't want to engage'," Mr Topic said.

She worked at the local supermarket and spent her spare time at home with her family.

"She struggled as time went on in social situations, she started to become more and more uncomfortable with people around, she was having difficulty looking people in the eye," Ms Topic said.

"She would just be in her room for hours [but] she was always included in everything we did; every time we left the house for a family function she was always there."

'Five, six cops and one lady standing in the middle of them'

Sorry, this video has expired Ashton Tuimaseve describes witnessing the shooting death of Courtney Topic

Courtney's parents said she would never go out on her own, so no-one can explain what happened that day.

She took a large kitchen knife from her home and headed to the nearby shops.

Her mother says there was no warning.

"If we thought there was anything wrong that morning we would have all been here," Ms Topic said.

"There was no inkling that anything was going to transpire."

Police received calls about an agitated young woman in the car park of a fast food outlet, holding a knife and acting oddly.

Ashton Tuimaseve was driving past and started filming on his mobile phone.

"Five, six cops and one lady standing in the middle of them," he said.

"She just stood there and they were yelling at her, yelling orders at her.

"She had no emotion on her face.

"The next thing you know she's taken to one of the cops and he's pulled the trigger."

Other witnesses said that after a confrontation with police, Courtney had lunged towards a police officer.

'I hope there are some answers that will give us some peace'

Courtney Topic with her father Ron (l), mother Leesa (r) and three brothers ( Supplied: Leesa Topic )

Courtney's family still can't understand how it ended that way.

"I feel it could have been dealt with in another manner," Ms Topic said.

"Our understanding is it was over very quickly … and from the little bit I've seen online it breaks my heart 'cause my little girl looks so scared."

The inquest is expected to be told that police tried to taser Courtney, but the device failed; then another officer used capsicum spray.

Moments after, Courtney was shot.

Mr Topic believes things could have ended differently with a different police response.

"Surely we can do something better, make steps, we need to make changes," he said.

Ms Topic said: "I hope the Coroner can come up with some answers that will give us some peace.

"It won't bring Courtney back, but we will have a better understanding of what happened and things that may be done differently if, God forbid, this scenario every transpires again."