Carroll used a steering wheel lock to hit Tucker on the head a number of times, resulting in severe head injuries.
"I picked up the steering wheel lock and hit him in the shoulder... I hit him again in the shoulder, he went floppy and stopped moving," Carroll told the High Court at Christchurch on Tuesday.
Carroll and Tucker were both involved in the city's drug scene, and the court heard that Tucker had been using fake $100 notes to buy drugs days before his death.