New York: The Minneapolis police officer who shot dead Australian Justine Ruszczyk Damond should not have used any force - even pepper spray or a taser, let alone a gun - when she approached his police car, an expert witness has testified in court.

Minneapolis police officer Mohamed Noor, centre, has been charged with two counts of murder and one of manslaughter for killing Justine Ruszczyk Damond. Credit:ninevms

Use-of-force expert Derrick Hacker told the court on Wednesday local time that just because Mohamed Noor, the officer who shot Ruszczyk, felt afraid did not mean he was justified in shooting an unarmed civilian.

Noor, 33, has been charged with two counts of murder and one of manslaughter for killing Ruszczyk in July 2017. The 40-year old from Sydney's northern beaches, who had called police to report a possible sexual assault near her home, was unarmed and wearing pyjamas at the time.

Hacker said Noor's actions were "excessive, objectionable, unreasonable and extremely dangerous".