Police officer appears in US court to explain why he shot dead Australian Justine Damond

A former Minneapolis police officer on trial in the fatal shooting of an unarmed Australian woman has testified that he saw a woman in a pink shirt with blonde hair at his partner's window, raising her right arm, before he fired his gun "to stop the threat".

Mohamed Noor refused to talk to investigators after the July 2017 shooting of Justine Ruszczyk Damond, a dual citizen of the US and Australia, making his testimony his first public statements since her death.

Former Sydney resident Ms Damond had called 911 minutes earlier to report a possible sexual assault behind her home.

She was shot as she approached Mr Noor's squad car as he and his partner slowly rolled down her alley looking for evidence of a woman in distress.

Former Minneapolis police officer Mohamed Noor on the first day of trial in Minneapolis. Source: AAP

Mr Noor testified that he fired to stop what he thought was a threat to him and his partner, Matthew Harrity, after he heard a loud bang on the driver's side of the squad car.

Mr Noor said he saw fear in Mr Harrity's eyes and saw that he was trying to pull his gun but was having difficulty.

He described putting his left arm over Mr Harrity's chest, and seeing a woman in a pink shirt with blond hair outside Mr Harrity's driver's side window raising her right arm.

"I fired one shot," Mr Noor said. "My intent was to stop the threat."

Justine Ruszczyk Damond, 40, was a dual citizen of the US and Australia. Source: Getty

When he realised he had shot an innocent woman, Mr Noor said, "I felt like my whole world came crashing down."

"I couldn't breathe ... I felt great pain."

Prosecutor Amy Sweasy attacked Mr Noor in cross-examination, noting that he didn't see Ms Damond's hands or a weapon.

"You meant to shoot the woman to stop the threat?" she asked "You knew you were shooting a person?"

"Yes ma'am," he answered.

This courtroom sketch depicts former Minneapolis police officer Mohamed Noor, centre, on the witness stand in Minneapolis. Source: AAP

Mr Noor was fired from the force soon after being charged.

His lawyers have said he was spooked by a noise on his squad car right before the shooting and feared an ambush.

The death of Ms Damond, a 40-year-old life coach who was engaged to be married a month after her death, sparked anger and disbelief in both the US and Australia. It cost the city's police chief her job and contributed to the mayor's electoral defeat a few months later.

Justine Damond supposedly banged the car, but no fingerprints

Prosecutors have questioned the supposed noise, presumably from Ms Damond slapping the car as she approached, by noting that investigators didn't find forensic evidence of her fingerprints on the car.

A march in honour of Justine Damond in Minneapolis. Source: AAP

They also questioned the timing of Mr Harrity's first mention of the thump - not the night of the shooting but a few days later, as he was being interviewed by state investigators.

Neither officer had their body cameras running when Ms Damond was shot, something Mr Harrity blamed on what he called a vague policy that didn't require it.

The department toughened the policy after Ms Damond's death to require that the cameras be turned on when responding to a call.

Mr Noor and his attorneys are surrounded by media as they enter the Hennepin County Government Centre in Minneapolis. Source: AAP

Ms Damond was white. Mr Noor, 33, is a Somali American whose hiring two years before the shooting was celebrated by Minneapolis leaders as a sign of a diversifying police force in a city with a large population of Somali immigrants.

Mr Noor testified earlier on Thursday about immigrating from Somalia to the US, where he became a citizen in 1999.

He lived first in Chicago, then moved to Minneapolis, where he said he fell in love with the city.

He said he became a police officer because he "wanted to serve".

Do you have a story tip? Email: newsroomau@yahoonews.com.

You can also follow us on Facebook, download the Yahoo News app from iTunes or Google Play and stay up to date with the latest news with Yahoo’s daily newsletter. Sign up here.