He claims that it was Mngeni - also known as Watti - who shot Mrs Dewani.
In his account, Qwabe said the shooting took place after Mr Dewani and Tongo were dropped off.
"While I was driving the vehicle Watti shot and killed the deceased. Watti was seated in the front passenger seat at the time that he fired the single shot."
He said he became "scared and nervous" and pulled over at the side of the road, where he helped his alleged accomplice find the bullet casing.
Qwabe said he threw the casing into a storm drain as the pair ran away. He said he had worn yellow gloves during the episode to avoid detection.
Days later he was arrested and confessed his involvement to the police, helping them to find the gloves, the casing and the pistol used in the murder.
Qwabe also admitted to kidnapping, firearm possession, and robbery of Mrs Dewani's Giorgio Armani ladies' watch, white gold and diamond bracelet, handbag and Blackberry, worth a total of 90,000 Rand (£7,000).
He said he and Mngeni agreed to be paid 15,000 Rand (£1,200) for the killing.
Tongo has already admitted his role and told prosecutors that Dewani ordererd the carjacking and paid for the hit.
The plea deal came ahead of Qwabe and Mngeni's trial, due to start on Monday.
Prosecution spokesman Eric Ntabazalila said: "It's a positive step for the prosecution because we are now a step closer to getting information on what happened on that day, which will shed some light on the role played by Shrien Dewani."
The victim's uncle Ashok Hindocha welcomed the development, saying: "It is a legal step forward, now two of the men have admitted the crime and are sentenced.
"That is a little positive but the main question here is, what happened to Anni? We would like to know. I hope the case gets a little further now he has admitted he was involved. It is a step forward."
Mr Hindocha said the lengthy court process was "legal torture" for the family and had affected the health of Anni's parents.
"We are all under enormous psychological stress," he said.
"Every time there is some kind of hope we might get answers, something happens and back to square one."
Dewani, 32, a Bristol care home owner, has denied hiring anyone to kill his wife.
He faces extradition to South Africa but proceedings were halted earlier this year because of his poor mental health.
His lawyers in the UK have asked for the process to be held up for 12 months for him to recover from depression and post-traumatic stress disorder. A further hearing on his case is due next month.
Dewani is under medical treatment after being sectioned.
His brother Preyen declined to comment on the latest development.