Mohamed Noor has not said a word until now.

Key points: Mohamed Noor is on trial for the alleged murder of Sydney woman Justine Damond Ruszczyk

Mohamed Noor is on trial for the alleged murder of Sydney woman Justine Damond Ruszczyk Ms Damond Ruszczyk called police because she suspected a sexual assault was happening outside her home

Ms Damond Ruszczyk called police because she suspected a sexual assault was happening outside her home Mr Noor told the court he thought Ms Damond Ruszczyk was a "threat" to him and his partner

The former Minneapolis police officer who is accused of killing Australian Justine Damond Ruszczyk has been silent in the months since the July 2017 shooting.

He has so far refused to talk to police investigators or provide Ms Damond Ruszczyk's family with an explanation.

In a surprise development in court on Friday, defence counsel Tom Plunkett announced Mr Noor would be the first witness for the defence.

His testimony detailed the moments of a killing the prosecution says is murder and the defence calls a tragic mistake.

Mr Noor describes the night of the shooting

Mr Noor recalled seeing a blonde female in a pink T-shirt approaching the squad car on the night of the shooting.

"My intent was to stop the threat," Mohamed Noor told the court. ( AP: Cedric Hohnstadt )

She moved her right arm and his partner yelled 'oh Jesus' after he heard a thump on the car, Mr Noor said, naming those two actions as the reason he shot his gun.

He also said that the gun of his partner, Police Officer Matthew Harrity, appeared to be stuck in its holster.

Mr Harrity had fear in his eyes, Mr Noor said.

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

Defence counsel Tom Plunkett led him through that decision to fire.

Noor: "She could have had a weapon, sir." Plunkett: "Why didn't you wait?" Noor: "My partner would have been dead."

Loading

Under cross-examination by the prosecution, Mr Noor admitted he couldn't see Ms Damond Ruszczyk's hands.

Prosecutor Amy Sweasy tried to discredit Mr Noor's description of a threat.

"Her whole blonde hair, pink T-shirt and all — that was all threat to you?" she asked.

Upon realising that he'd shot an unarmed woman, Mr Noor said he felt paralysed and struggled to breathe.

"It felt like my whole world came crashing down," he said.

Defence draws out Mr Noor's backstory

Mohamed Noor said he became a police officer to serve the city of Minneapolis. ( AP: City of Minneapolis )

The defence team tried to soften Mr Noor for the jury by asking about his upbringing in Somalia, his move to Chicago and subsequent move to Minneapolis in high school.

The defence painted a picture of a young man who was the eldest of 10 siblings and overcame those initial obstacles in Minneapolis.

"My peers [in Minneapolis] accepted me, but when I moved here, no-one liked Somalis and I picked that up right away," he said.

After obtaining dual degrees in business administration and management, he was working as a pharmaceutical analyst before deciding to switch careers and applied for a job with the police department in 2015.

"I always wanted to serve, primarily the city of Minneapolis and the diverse community there," he said. "I fell in love with the city."

However, later in his testimony he would confess he never foresaw the tragic end of his police career.

"If I knew this would happen, I never would have become a cop," he said.

Mr Noor details counter-ambush training

Justine Damond Ruszczyk called the police because she thought a sexual assault was happening outside her home. ( Linkedin: Justine Ruszczyk )

The defence team questioned Mr Noor on his 29 weeks of training as a police cadet, including the use of force.

Mr Noor described "counter-ambush" training that included scenarios such as two officers in a squad car, doing routine tasks, and an instructor yelling "threat!". The officers had to make a quick decision about whether to shoot, Mr Noor said.

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

Mr Noor described another training exercise, for which he was sent to a location, heard gunshots and instead of assessing the threat, he ran toward it. An instructor shot him with a paintball gun, he said.

"So the point is if you don't do your job correctly, you'll get killed," Mr Plunkett said.

"Yes sir," Mr Noor answered.

Mr Noor's attorneys suggested that Mr Noor may have feared an ambush in the alley on the night Ms Damond Ruszczyk was killed.

Judge Kathryn Quaintance pushed back saying there was no evidence this was an ambush.

Ms Quaintance has previously ruled the defence cannot refer to previous well-publicised police ambushes, though the defence has tried to get around it in examination of witnesses.

The defence may have hoped that calling Mr Noor later in the day shortened the window for prosecutorial cross examination.

However, it will now spill in to Saturday, giving prosecutors Amy Sweasy and Patrick Lofton time to work on a strategy.

Earlier, Judge Quaintance ruled the prosecution could not submit evidence like Mr Noor's police psychological evaluation or a video that shows Mr Noor pointing a gun at a motorist during a traffic stop.

While the prosecution has rested its case, it may seek to have this evidence readmitted.