The first-class constable faced a three-week trial in the WA Supreme Court over the September 2019 killing of the 29-year-old woman, known as JC for cultural reasons, in the state's Mid West.

After deliberating for three hours on Friday the jury found the officer, who cannot be named for legal reasons, not guilty of JC's murder.

The jury also returned a not guilty verdict to the lesser charge of manslaughter.

JC, a mother-of-one, had experienced significant mental health and drug problems and recently been released from prison when she was shot dead in Geraldton.

The accused was one of eight police officers who swarmed on the street where JC was seen carrying a large knife and a pair of scissors.

Five of the officers stayed in their cars, a sixth left his vehicle unarmed and attempted to speak to JC, and another drew his Taser but did not activate it.

The court heard the accused had gotten out of his vehicle, drawn his loaded firearm and ran towards JC before firing a single shot at her abdomen from close range.

He testified he had acted in self-defence, claiming JC had turned towards him and raised the knife before he pulled the trigger.

Defence lawyer Linda Black told the court JC had ignored repeated requests to drop the knife from the officers at the scene.

The jury was shown CCTV footage, taken from a home about 65 metres away, of JC being shot while surrounded by police vehicles.

Director of Public Prosecutions Amanda Forrester SC argued the footage showed JC did not move towards the officers.

The accused had spent five years as a permanent officer and 18 months as a probationary constable prior to the shooting.

Given his extensive training, the number of officers present and the other available weapons, the shooting was "wholly unnecessary", Ms Forrester said.

Ms Black said her client had acted correctly by drawing his gun, rather than a Taser, when confronting a person armed with a knife.

She said the officer had never fired his gun while on duty and had less than a second to decide whether to pull the trigger given the close proximity.

"He was not some trigger-happy constable ... he was a brave and careful officer who took pride in his job," Ms Black said.

"(JC) was never, ever going to drop the weapons. She needed to be taken down; she was never going to surrender."

The trial heard JC had battled serious substance abuse issues and did not have custody of her young son, which was a source of distress.

Ten days prior to her death, she had been hospitalised after calling triple zero and saying she was going to kill herself.

Australian Associated Press