An appeal has succeeded in nearly doubling the prison sentence given to Sydney man Kieran Loveridge for the one-punch death of teenager Thomas Kelly and several assaults on the same night.

Mr Kelly, 18, died after being randomly punched by Loveridge as he walked with his girlfriend in Kings Cross in July 2012.

Loveridge was sentenced last November to a minimum of five years and two months in prison over that attack and three other random assaults on the same night.

After a community backlash, the New South Wales Director of Public Prosecutions (DPP) launched an appeal against the leniency of the sentence.

A three-judge panel in the NSW Court of Criminal Appeal this morning quashed the original sentence.

Loveridge will now serve at least 10 years and two months in prison, while the maximum jail term is now 13 years and eight months.

Mr Kelly's mother cried quietly in court as she listened to the panel's decision.

Outside the court the dead teenager's family welcomed the new sentence.

Sorry, this video has expired Kelly family speaks after Loveridge's sentence increased

Stuart Kelly, who was 14 when his older brother died, spoke publicly about his loss for the first time.

"I now no longer have an older brother. Instead I have a hole in my life that somehow I have to come to terms with and accept," the 16-year-old said.

"Up until now I haven't said anything because I was too young. I've just tried to block it out. I've tried to understand over and over why, at such a young age, my brother lost his life so tragically, but I have no answers.

"It's been difficult for me. I've watched my family suffer on a daily basis, trying to somehow regain anything in their lives.

"I want Thomas's short life to have some meaning in his death so we can see change, a new fresh start, one where I believe we should have respect for one another."

Family and friends of Thomas Kelly, including his father Ralph (front left), sister Madeleine (back left), brother Stuart (centre) and mother Kathy (2nd from right) outside the court ( AAP: Dean Lewins )

Father Ralph Kelly said the appeal's success is bittersweet, coming three days short of the anniversary of his son's death.

"There is no celebration in this and you can't value a life in years in jail," he said.

"But we acknowledge the court system and we respect that finally today they acknowledged Thomas, which is fantastic to see in some respects.

"It's a stress that no family should have to take - emotionally, physically, mentally - every kind of stress.

"It shouldn't be that the victims have to go through this. It should be the offender should be given a sentence that is decent and let them appeal."

Stronger sentence 'to deter alcohol-fuelled violence'

The minimum sentence for the manslaughter of Mr Kelly alone has been increased from four to seven years.

Thomas Kelly died in hospital two days after being randomly punched at Kings Cross. ( Supplied )

The court heard the original sentence did not act as a strong enough deterrent to alcohol-fuelled violence.

"In upholding the Crown appeal the court held that the sentencing judge had a made a number of specific errors," a summary of the judgement by Chief Justice Tom Bathurst, Justice Peter Johnson and Justice Robert Hulme said.

"The first error found was a failure to take into account the need for general deterrence for this kind of offending.

"Violence on the streets, especially by young men under the influence of alcohol, is all too common."

NSW Attorney-General Brad Hazzard released a statement welcoming the new sentence.

"The NSW Government and the community wanted the sentence appealed and we thank the Office of the Director of Public Prosecutions for its work on the appeal," Mr Hazzard said.

"The NSW Government has since mandated minimum jail sentences for these kinds of deadly, alcohol fuelled assaults and has put in place lockouts to change the culture of drunken, dangerous behaviour in central Sydney."

Today's sentence for the manslaughter of Mr Kelly still falls short of the eight-year mandatory minimum sentence now in place for fatal one-punch assaults in public where the offender is drunk or on drugs.