"I don't know who shot him," another yelled as other police scrambled behind a wall to take cover after five rapid gunshots.

"He's dead mate, we can't get in there to help, Crews is dead," one cried.

For the next 19 minutes, video footage captures members of the Middle Eastern crime squad screaming out as their colleague Detective Constable William "Bill" Crews lay motionless and bleeding on the floor of a Bankstown garage.

The footage of confused and distressed officers was played during an inquest into the death of Constable Crews, who was mistakenly shot by another officer during a drug raid in September 2010.

The inquest, which began on Monday at Glebe Coroners Court, will look at the mistakes made in a risk assessment that classified the operation as "low" and why police ran in the wrong direction when they entered the Cairds Avenue unit block.

It will also look at why the search warrant was rushed through on the ill-fated night and whether police training and procedures were adequate.

Constable Crews' parents, Kelvin and Sharon, watched the final moments of their son's life during the inquest with their other son Ben Crews.

On the the night of September 8, police barged down the stairs of a unit block after receiving a tip-off that drug dealer Phillip Nguyen, 58, was selling cocaine and heroin out of a garage to Middle Eastern crime families.

The plan was to raid garage No.8 and a unit upstairs at the same time, both of which could only be accessed though the basement. But confused police ran in the wrong direction once inside the building.

Within seconds of police entering the garage, Nguyen fired a shot. The bullet hit Constable Crews in his upper left arm.

Four more gunshots echoed throughout the dimly lit basement within seconds.

Detective Senior Constable Roberts returned fire to protect himself and his colleagues but his gun was mistakenly directed at Constable Crews.

One shot penetrated his neck and killed him.

Counsel assisting the coroner, Dean Jordan, said the footage showed it was clear the tragic circumstances surrounding the slain officer's death "unfolded very quickly indeed".

"It is clear that there was confusion in relation to the location of unit eight and the garage," he told the inquest.

"There was a mistake in relation to directions. The officers headed to garage one, where the tragic events occurred, instead of to the left, where garage eight was."

After the flurry of gunshots, police took cover behind a wall before they realised Constable Crews was not with them.

They waited there for 19 minutes under the mistaken belief that Nguyen was still armed around the corner but he had fled upstairs to his apartment, where his wife and children were.

Nguyen is serving a maximum 16-year sentence for the manslaughter of Constable Crews.

The inquest heard that even if Constable Crews had received immediate medical help, it was unlikely he would have survived.

An officer who carried out a risk assessment before the raid said, with the benefit of hindsight, the operation should have been classified as "high".

At the time, Detective Senior Constable Richard McNally told the inquest he believed it was a "low risk" operation and that the offender was unlikely to be armed.

He now believes several mistakes were made in the risk assessment and said he would now consider the possibility of other people being present during a raid involving a drug dealer.

The court heard police had information to suggest Nguyen was selling drugs to the well-known Hamzy and Kalache families, who were known to have access to firearms.

The inquest continues.