Media playback is unsupported on your device Media caption Justine Damond's family held a silent dawn vigil on Sydney's Freshwater Beach

The lawyer for a US police officer whose partner killed an Australian woman says it would be "reasonable" for the pair to have feared an ambush.

Minneapolis officer Matthew Harrity has reportedly said they were startled by a "loud sound" before last Saturday night's shooting of Justine Damond.

Police have released the transcript of her call to police, in which the 40-year-old reports a suspected rape.

She was fatally shot in the abdomen by one of the officers she had called.

Officer Mohamed Noor, who fired the fatal shot in Ms Damond's upmarket neighbourhood, has refused to be interviewed by investigators, as is his legal right.

Fred Bruno, a lawyer for Officer Harrity, said on Wednesday: "It is reasonable to assume an officer in that situation would be concerned about a possible ambush.

Media playback is unsupported on your device Media caption Australian PM Malcolm Turnbull: 'We're seeking answers'

"It was only a few weeks ago when a female NYPD cop and mother of twins was executed in her car in a very similar scenario."

He was referring to the 5 July shooting of a 48-year-old police officer as she sat in her patrol car in the Bronx borough of New York City.

The attorney's comments come a day after Officer Harrity spoke to investigators with the state's Bureau of Criminal Apprehension, which is leading the investigation.

During the interview, he described seeing a young person on a bicycle pass by moments before Ms Damond pounded on the door of the police car, according to KSTP-TV.

Detectives have appealed to the cyclist to come forward with any information he may have.

Media playback is unsupported on your device Media caption "Justine should be here. This shouldn't have happened"

On Wednesday police released the transcript of her two separate 911 calls, which she made after hearing screams nearby.

"I'm not sure if she's having sex or being raped," she told the police operator, before giving her address.

"I think she just yelled out 'help', but it's difficult, the sound has been going on for a while," she continued.

Ms Damond called back eight minutes later to ensure police had the correct address.

Body cameras, which are worn by all Minneapolis police, had not been turned on at the time of the shooting and the squad car dashboard camera also failed to capture the incident.

Image copyright EPA Image caption Friends held a vigil at Sydney's Freshwater beach on Wednesday morning

Officers Harrity and Noor, who between them have spent three years on the police force, have been placed on paid administrative leave.

Australian Prime Minister Malcolm Turnbull is appealing to the US for an explanation.

"It is a shocking killing, and yes, we are demanding answers on behalf of her family," he told Australian TV on Wednesday.

Hundreds of friends and family of Ms Damond held a vigil on Sydney's Freshwater beach on Wednesday morning.

The slain yoga instructor and spiritual healer was engaged to marry an American man.

Minnesota Governor Mark Dayton told reporters he has been in touch with the Australian embassy, adding the state may need to review rules covering police use of body cameras.