Probationary Constable Angela Tyson broke down saying she had no choice but to "engage" and approach the woman who was standing near a busy car park in front of Hungry Jack's.

"I felt it was totally justified ... She wouldn't put the knife down and then she started going for (Senior Constable Ethan Tesoriero)," she told Glebe Coroners Court on Friday.

The inquest is investigating the death of Courtney Topic, 22, who was shot by an officer at West Hoxton in February 2015.

Prob Const Tyson, who had her Taser pointed at the woman, said although she told her to drop the knife several times she didn't respond.

"She took steps towards me and that's when I fired ... I thought there was an immediate threat to my life," she said.

The inquest was told the officer pulled the trigger of the Taser, which had not been fully tested for six years, but it did not fire.

The coroner has heard Ms Topic had probably been experiencing a psychotic episode for an extended period and likely suffered undiagnosed schizophrenia.

The Topic family lawyer, Sophia Beckett, suggested that the "shocking" event had been "escalated" by the police actions and led to Ms Topic running from them, rather than at them.

None of the police messages had communicated that Ms Topic was violent and footage showed her standing still with her arms down as the officers approached, the lawyer said.

But Prob Const Tyson said Ms Topic had pointed the knife at her as she began to run.

"That all took place in about one second, there was no time for us to do any other thing," she said.

"Unfortunately de-escalation tactics" wouldn't have worked.

Police radio calls from the day reveal responding officers were told the "distressed" woman was "talking to herself" and pointing a large carving knife at her stomach.

Officers also heard the young woman in a striped top and jeans was "waving" the knife about and "hitting herself in the head".

The mobile data terminal in Prob Const Tyson's police car also displayed mental health warning codes beside the incident details.

She said that although she'd "probably" heard and read most of the multiple police messages about Courtney Topic she remained focused on the knife.

The officer, who had been on the job for just four-and-a-half months, said she had not received mental health intervention training.

She said her interpretation of police radio and data messages was that the young woman was "drug-affected or intoxicated".

The inquest continues.

Australian Associated Press