His accomplices Ben Purua, then 15, and William Frederick Izett, then 17, had pleaded guilty to manslaughter and were sentenced to 5 and four years' jail respectively. They will each serve a minimum of half of their sentences.
In front of a gallery packed with family from both sides, Mrs Watson told the court she would never understand how someone could consciously end another person's life.
Mr Stewart's body was found by a street sweeper in a central-city street the morning after an All Blacks test.
It was later revealed the three teenagers were drunk or high on cannabis or methamphetamine when they came across the Lower Hutt man's car parked near public toilets with the headlights on.
They tried to steal the car but found it locked.
When Mr Stewart returned, they demanded his keys, but Mrs Watson said her father loved his car, his laptop and photographs that were in it and would never have given up his prized possessions.
So Rewha-Te Wara punched him in the head and knocked him to the ground as the other two looked on.
Mr Stewart was dragged to an alleyway where Rewha-Te Wara meted out what Justice Patrick Keane described as a brutal, cruel, depraved and callous assault during which one blow to Mr Stewart's head was so strong it ruptured an artery.
The retired draughtsman and builder originally from Taumarunui was left for dead as the trio took his 1989 Peugeot 405 for a joyride before crashing it into a ditch outside the city.
Mrs Watson said her father's "horrendously violent" death had devastated her extended family, including her older sister Stephanie Fallon, and she had lost confidence in people, felt unsafe in New Zealand and was exhausted through a lack of sleep, plagued with nightmares.
"I feel very alone in my sadness."
Her children now had a "sense of anxiety and fear about them and found it very difficult to understand why someone would do this to their granddad".
Both Rewha-Te Wara and Purua came from large families with gang affiliations and were raised in domestic violence while Izett was adopted as an only child but had a history of convictions.
Justice Keane told Rewha-Te Wara, a member of The Bloods youth gang at the time of the murder, that he hoped the teen would come to understand the gravity of his heedless actions, which had blighted the lives of Mr Stewart's family.
Outside the court, some members of the prisoners' families hugged some of Mr Stewart's family amid tears.
Brother Gordon Stewart said the family were working to forgive the teenagers and they hoped the trio would turn their lives around in jail.
Detective Senior Sergeant Mark Greene described Mr Stewart as a true gentleman who didn't deserve to have his life ended that way.
