Stuart Kelly's haunting words to his brother's killer months before his own death

"I carry a deep scar that you cannot see. It's always there, never leaves. It's just below the surface of your skin, and surfaces when you least expect it," Stuart Kelly said in the aftermath of his older brother's death from a drunken punch in 2012.

Now, the younger brother of Thomas Kelly has also died, visiting a new wave of grief upon his already shattered family.

Stuart's words echoed across the country when he appealed to Australians to rethink their relationships with alcohol, and to the NSW government to bolster laws against violent, drunken people.

"I ask all of you to look at me, I am but one person who has been affected by violence," Stuart said in September last year.

"It is a sentence that I have to carry for the rest of my life.

"My mother, father and sister now carry this sentence. Our relatives and friends, Tom's friends, carry this sentence.

"We are not alone, there are many many thousands of other who are directly affected by senseless violence every year.

The younger brother of one-punch victim Thomas Kelly has died, delivering another heart-wrenching blow to the Kelly family four years after the passing of their eldest son.

19-year-old Stuart passed away in Mona Vale on Monday, four years after his family saw his older brother's life support switched off in a Sydney hospital.

The principal of the brothers' old school, King's School in Parramatta, announced the news to his students.

Headmaster Dr Tim Hawkes said thoughts and prayers were extended to the Kelly family.

“It is with great sadness that I inform you of the death of an Old Boy of the School, Stuart Kelly. This is the second great tragedy to affect the Kelly family," he said.

“The exact circumstances surrounding his death are not known, however, is it enough to know that we have lost a member of our community…”

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Both brothers were former pupils at the school. Their sister, Madeleine, is now the only surviving sibling in the family.

The cause of his death has not been revealed however police said it was not being considered as suspicious.

"Police will prepare a report for the coroner following the death of a 19-year-old-man at Mona Vale yesterday," a statement read on Tuesday.

"There are not believed to be any suspicious circumstances."

Stuart was only 14 years old when his big brother Thomas, 18, was fatally punched in Kings Cross in 2012.

After his brother's death, Stuart rallied against violence and gave an emotional speech at an event last year where he recalled the moment doctors told him Thomas’ life support would be switched off.

Thomas' attacker Kieran Loveridge was sentenced in 2013 to four years behind bars for his manslaughter charge and a further one year and two months for other offences committed on that night.

"Thomas Kelly was the victim of an unprovoked attack, and I have great sympathy for his family who are devastated by their loss,” NSW Attorney-General Greg Smith said in 2013.

"Drunken assaults are a terrible scourge, and every weekend we hear about attacks by intoxicated irresponsible people on bystanders who are lucky to escape with their life."

Kathy Kelly cried when she heard the verdict, saying the penalty was not enough for her son’s life.

"Four years for your son's life," Kathy Kelly said. "We're horrified. Absolutely horrified. Somebody else will be standing here in a few months or a year's time like we are and be heartbroken.

"How many boys or how many of our children have to die before somebody does something to change these laws to make people accountable for what they do?"

After Thomas’ death, the Thomas Kelly Youth Foundation was formed to change Australia’s drinking culture through education.

Stuart's passing comes three weeks after the fourth anniversary of his Thomas' death.

If you are concerned about the mental health of yourself or a loved one, seek support and information by calling Lifeline 13 11 14, Mensline 1300 789 978