The driver in a fatal car accident asked police to shoot him when he realised his friend had died, the ACT Supreme Court has heard.

Jamie Richardson, 24, has been on trial for culpable driving after crashing a car in Canberra's north in February 2014, killing his friend Glenn Dunster and badly injuring a female passenger.

He admitted he was drunk but maintained one of the passengers grabbed the steering wheel and that was what caused the car crash.

Constable Nathan Macklin, who went to the scene, told the court at first Richardson indicated someone else was the driver and they had run away.

The court heard when Mr Dunster's condition worsened and Constable Macklin became concerned about the state of the driver, Richardson said: "I was the driver, stop looking."

Richardson was then cautioned and a breathalyser test was carried out.

Prosecutor Trent Hickey asked Constable Macklin how Richardson reacted when a blanket was placed over his dead friend.

"He punched himself in the head and yelled something I do not recall," he said.

"He was very, very hostile to everyone who was around, he was angry, he was upset.

"He said, 'My life is over, just shoot me in the head, just do it, I've killed him'."

Under cross-examination, Richardson's lawyer challenged his account, suggesting he was not hostile but was very upset because his friend had died.