FIRST ON 7: A policeman has explained to the Coroner's court why he shot a man, who was sitting in a car on a Castle Hill street.

Detective Paul Gardiner walked investigators through the scene where he shot a man in a ute in Sydney’s north-west.

“I've brought my firearm up and fired. twice,” Gardiner said.

The Senior Constable had arrived early for his shift on September 29 last year, when he heard a call on the police radio about a man waving a gun.

He and two other plain clothes officers went running and minutes later, Rodney Elkass was dead from a bullet wound to the head.

Detective Gardiner today told an inquest of his split second decision to open fire, saying he and a colleague did so to save another officer, after Elkass pointed a gun at him.

“[He] came out and just went like that, straight at Paul,” Gardiner said.

Thirty-seven witnesses have said they didn't hear anyone identify themselves as police, but Detective Gardiner today insisted that he did.

"I said it at least once, I am sure I said police."

The inquest is considering whether plain clothes officers should put on fluro caps or vests when responding to emergencies.