Having already served a significant part of his sentence, and with no non-parole term imposed in court yesterday, he is now eligible for parole.
He is likely to be released from prison by the end of next year.
Justice Dunningham set a starting point of eight years' imprisonment, with an uplift of six months for significant anti-social behaviour and ongoing offences on remand.
He was given a 33 per cent discount for his youth.
Justice Dunningham said Smith had demonstrated oppositional behaviour from a young age, despite a stable home background and a good relationship with his parents.
He had been expelled from Waitaki Boys' High School in his early teens and by age 16, he had 43 previous youth court matters, of varying degrees of seriousness, including common assault, robbery and dishonesty offences.
He also had issues while on remand before his first trial in 2011, having to be moved to a different corrections facility, with 39 reports of incidents, and had even recently demonstrated aggressive behaviour to staff in Christchurch Men's Prison when he did not get his own way, Justice Dunningham said..
A pre-sentence report indicated his likelihood of re-offending was high.
Smith's defence had argued he slashed William's front and stabbed him in the back three times with a hunting knife in self-defence.
At a retrial by jury last month, self-defence was rejected outright because of the excessive force used to stab William, but the six men and six women of the jury were left in reasonable doubt in terms of murderous intent.
Justice Dunningham said she was satisfied that there was only a limited argument for self- defence.
Five members of William's family read their victim impact statements aloud in court on Friday, expressing how traumatised they had been by the unexpectedness and brutality of his death.
"There can be no doubt that they are deeply grieved and scarred for life," Justice Dunningham said.
Perhaps the most moving tribute was that of William's step-brother Jacob Lewis, she said, who told Smith and the court how he looked up to his big brother -- the best person he had ever met.
As he had to say good-bye to his big brother forever, "no one could understand what I was feeling".
He described his big brother as a "gift to all of us".
"[William] loved me, played with me and always knew me better than I knew myself," he told Justice Dunningham.
"I wish there was some way to bring him back." The family expressed their doubt that Smith showed any remorse and Justice Dunningham told the 20-year-old if he could do one thing to express it, it could be to live a good and productive life from this day forward.
"The best way to make up for what you have done would be to commit to a more settled lifestyle -- I hope you can achieve that," she said.
The Crown submitted a starting point of nine to 10 years' imprisonment was appropriate, recognising "extreme violence", premeditation and the use of a weapon.
Crown prosecutor Andrew McRae said Smith, at age 16, was no stranger to the authorities and on the night of April 1, 2010, no real threat existed towards him -- at worst, a punch.
Mr McRae said Smith sought permission to fight via a text to his girlfriend and he told her he was "strapped" with a knife -- all indications he was prepared to fight and use the knife.
Evidence given by William's best friend, Donovan Smith, indicated he was turning to move away when Smith grabbed his shoulder and stabbed him three times in the back.
Texting his girlfriend in the aftermath, Smith told her he stabbed William out of anger after an argument with his father at work earlier that day, Mr McRae said.
"He also did nothing to assist the victim ... he did everything he could to distance himself from the situation," Mr McRae said.
There was a degree of premeditation as the knife was not something he had grabbed and used.
It was on his person.
Mr McRae said William's death highlighted the danger of youths carrying knives.
"There's a need to deter not just the prisoner, but also in terms of the general public," Mr McRae said.
Defence counsel Christopher Stevenson said Smith reacted the way he did, feeling he was "under threat".
"He was a passive participant who only reacted after persistent and escalating intimidation," Mr Stevenson said.
Smith's reaction was excessive and his youth was relevant in the case, he said.
Smith had made clear offers at both trials to plead guilty to manslaughter and he should get full credit for that, Mr Stevenson told Justice Dunningham.
