The Minnesota Supreme Court has thrown out the third-degree murder conviction of a former Minneapolis police officer who fatally shot an Australian woman who had called emergency services to report a possible rape behind her home.

Justine Ruszczyk Damond’s fiance slammed the decision, branding it a “double blow against justice”.

In its ruling in the case of Mohamed Noor, the Supreme Court also clarified what would constitute third-degree murder, or depraved-mind murder, saying the statute did not apply if a defendant’s actions were directed at a particular person.

Noor was convicted of third-degree murder and second-degree manslaughter in the 2017 death of Justine Ruszczyk Damond, a dual US-Australian citizen who called police to report a possible sexual assault behind her home.

He was sentenced to 12-and-a-half years on the murder count but was not sentenced for manslaughter.

The ruling means his murder conviction is overturned and the case will now go back to the district court, where he will be sentenced on the manslaughter count.

He has already served more than 28 months of his murder sentence. If sentenced to the presumptive four years for manslaughter, he could be eligible for supervised release about the end of this year.

Justine Damond with fiance Don Ruszczyk and step-son Zach.

Ms Ruszczyk’s fiance, Don Damond, said the ruling was a “double blow against justice”, and that since her death, he had worked to try to prevent more fatalities at the hands of “stressed and inadequately trained police officers”.

He said the Minneapolis Police Department had not made any real progress towards change, and Noor was not being held accountable for his fiancee’s killing.

“I have lived with the tragic loss of Justine and none of this can hurt my heart more than it has been, but now it truly feels like there has been no justice for Justine,” he said.

Noor’s defence team released a statement, saying “fairness has been delivered” and Noor was looking forward to hugging his son as soon as possible.

Hennepin County Attorney Mike Freeman, whose office prosecuted the case, said he disagreed with the Supreme Court’s analysis but must accept the decision.

He said the second-degree manslaughter conviction remained and was just.

The ruling could give former Minneapolis Officer Derek Chauvin grounds to contest his own third-degree murder conviction in George Floyd’s May 2020 death. But that would not have much impact on Chauvin since he was also convicted of the more serious count of second-degree murder and is serving 22-and-a-half years on that count.

Experts say it’s unlikely Chauvin would be successful in appealing his second-degree murder conviction.

The Noor ruling was also closely watched for its possible impact on three other former Minneapolis officers awaiting trial in Mr Floyd’s death. Prosecutors had wanted to add charges of aiding and abetting third-degree murder against them, but that is unlikely to happen now.

The three men are due to go on trial in March on charges of aiding and abetting both second-degree murder and manslaughter.

-AAP