Justine Damond's killer has spoken for the first time about why he pulled the trigger.

With AAP.

It was after 11:30pm on July 15, 2017, when Justine Ruszczyk Damond approached the police car outside her Minnesota home. She was barefoot, wearing pyjamas and a pink koala t-shirt, clutching a glittery gold mobile phone in her hand. Within minutes, she was dead.

The 40-year-old was fatally shot by Mohamed Noor – one of the Minneapolis Police Department officers dispatched to respond to a 911 call she’d placed a short time earlier. The Sydney-born life coach, who was living in the US wither her American fiancé, had phoned police to report that she could hear what sounded like a woman being sexually assaulted in the alley behind their home.

Noor is currently on trial for murdering Damond, and gave surprise testimony on Thursday and Friday in which he spoke for the first time about why he pulled the trigger on the unarmed woman.

The 33-year-old claimed that when he and his partner, Officer Matthew Harrity, arrived at the residence, he heard a loud bang on his squad car. He testified that he then saw a woman appear at the driver-side window and raise her right arm. Noor told the court Officer Harrity had fear in his eyes, yelled “Oh Jesus!” and went for his gun but had difficulty pulling it out of the holster.

Noor believed he had to make a split-second decision. He pressed his left arm over Officer Harrity’s chest to protect him, and fired a single bullet through the open window, striking Damond.

“My intent was to stop the threat and save my partner’s life,” Noor told the downtown Minneapolis courtroom.

Noor’s decision to address the court came as a surprise to media reporting on the case. He had previously refused to talk to investigators or offer Damond’s family any form of explanation for his actions.

But during his testimony on Thursday, Noor spoke of "counter-ambush" training that included scenarios such as two officers performing routine tasks in a squad car and an instructor yelling "Threat!" The officers would then have to make a quick decision about whether to shoot, Noor said.

"Action is better than reaction," Noor said, referring to the training. "If you're reacting, that means it's too late to protect yourself. You die."

"Would you have discharged your weapon that evening if you were not concerned for your safety and your partner's safety?" Noor's lawyer, Tom Plunkett, asked in court.

Noor replied that he would not.

But on that July 2017 evening, that 'concern' cost Damond her life.

"I felt like my whole world came crashing down."

Noor recounted the moment he realised his split-second decision had resulted in the death of an innocent woman.

"I felt like my whole world came crashing down," he told the court. "I couldn't breathe."

He cried as he told the jury "he would never have become a cop" if he knew the incident was going to happen.

Still Noor, who was stood down from the force after the incident, stands by his response that evening. He has pleaded not guilty to charges of third-degree murder and second-degree manslaughter, which carry respective penalties of up to 25 and 10 years in prison.

The death of the Australian-American, who was engaged to be married a month after the shooting, sparked outrage in both the US and Australia, cost a Minneapolis' police chief her job and contributed to the electoral defeat of the city's mayor a few months later.

Damond's family filed a civil lawsuit against the city and several police officers last year seeking $US50 million ($A71 million) in damages.

The trial continues.