The 47-year-old who was suffering from chest pain, first called triple zero after pulling over on Monday afternoon on the Anzac Highway at Plympton.

After waiting 35 minutes, a bystander found the father in cardiac arrest and once again called triple zero, and his case was upgraded as a priority one, the state's Ambulance Employees Association said.

Bystander Josh Hamdorf started CPR however the man died shortly after the first paramedic arrived on scene.

"I didn't think twice," he said.

"I just went and did it and start doing CPR, it was a bit confronting because it was the first time I had ever done it."

More than 40 minutes after the initial triple-zero call, paramedics were unable to resuscitate him.

At the time of the case, the union said the ambulance service declared an "Opstat White", with 20 urgent cases left uncovered across the metropolitan area.