US prosecutors have laid a murder charge against a policeman who shot and killed an unarmed Australian woman.

Officer Mohamed Noor, 32, turned himself in over the death of Justine Damond in Minneapolis, prosecutors said. He is accused of third-degree murder and second-degree manslaughter.

Ms Damond died in July last year after calling police to report a possible sexual assault outside her home.

A lawyer for Mr Noor said his client had acted in line with police training.

But in announcing the charges, Hennepin County Attorney Mike Freeman alleged Mr Noor had "recklessly and intentionally fired his handgun".

"There is no evidence that Officer Noor encountered a threat, appreciated a threat, investigated a threat or confirmed a threat that justified his decision to use deadly force," Mr Freeman told reporters.

If convicted, Mr Noor could face up to 25 years in prison on the murder charge, and up to 10 years on the manslaughter charge.

The death of Ms Damond, also known as Justine Ruszczyk, caused an outcry in both the US and Australia.

Victim's last words
Mr Freeman said Mr Noor was sitting in the passenger seat of a police patrol car when he shot Ms Damond through a window.

After she was shot, Ms Damond put her hands on a wound to her abdomen and said, "I'm dying" or "I'm dead", according to Mr Freeman.

The incident prompted the resignation of Minneapolis police chief Janee Harteau last year.

It was described by Australian Prime Minister Malcolm Turnbull as an "inexplicable" and "shocking killing".

According to the third-degree murder charge, Mr Noor committed an "eminently dangerous act" while showing a "depraved mind".

The second-degree manslaughter charge accuses him of "culpable negligence creating unreasonable risk".

Mr Noor's lawyer, Tom Plunkett, said his client should not have been charged.

"The facts will show that Office Noor acted as he has been trained and consistent with established departmental policy," Mr Plunkett said in a statement.

The officer was placed on leave following the shooting, and elected not to speak to investigators.

At the time of the incident, neither Mr Noor or another officer present, Matthew Harrity, had their body-worn cameras activated.

Prosecutors had previously delayed a decision on whether to lay charges, saying they needed time to gather evidence.

Ms Damond's fiance, Don Damond, and her father, John Ruszczyk, said they welcomed the decision to charge Mr Noor.

"No charges can bring our Justine back," they said in a joint statement.

"However, justice demands accountability for those responsible for recklessly killing the fellow citizens they are sworn to protect, and today's actions reflect that."

Mr Noor is due to face a court in Minneapolis on Wednesday.