The US police officer accused of murdering Australian woman Justine Damond Ruszczyk outside her Minneapolis home is likely to plead not guilty, according to court documents.

The documents, filed by Mohamed Noor's lawyer, indicated the 32-year-old would fight the charges, arguing he acted in self-defence and used reasonable force.

Mr Noor shot Ms Damond Ruszczyk, a 40-year-old life coach, on July 15, 2017, after she called 911 to report a possible sexual assault in the alley behind her home.

Her death drew international attention, cost police chief Janee Harteau her job and forced major revisions to the department's policy on body cameras.

Mr Noor handed himself in to police in March on charges of third-degree murder and second-degree manslaughter.

He was booked on the third-degree murder charge for perpetrating an eminently dangerous act while showing a "depraved mind".

A second-degree manslaughter charge alleges he acted with "culpable negligence creating unreasonable risk".

Mohamed Noor had trained in business and economics and worked in property management before becoming an officer. ( Supplied: Hennepin County Sheriff's Office )

If convicted of third-degree murder, he could face a maximum of 25 years in prison, though the presumptive sentence is 12 and a half years.

The second-degree manslaughter charge carries a maximum sentence of 10 years in prison, but the presumptive sentence is four years.

Mr Noor has not spoken publicly about the case and declined to answer questions from investigators.

A policeman who was with Mr Noor at the time of the shooting, Matthew Harrity, told investigators that he was startled by a loud noise right before Ms Damond Ruszczyk approached the driver's side window of their police car.

Mr Harrity, who was driving, said Mr Noor then fired his weapon from the passenger seat.

Ms Damond Ruszczyk died of a gunshot wound to the abdomen.