Thomas' younger teenage brother and best friend Stuart Kelly, 18, was found dead on Sydney's northern beaches on Monday.

His death, which is not being treated as suspicious, comes almost three weeks after the fourth anniversary of Thomas' death.

Stuart was 14 when the older brother he greatly admired and saw as his closest friend was killed during a night out in Kings Cross.

Thomas, 18, was walking with his girlfriend along Victoria Street on July 7, 2012, when a heavily intoxicated stranger - Kieran Loveridge - punched him on the head.

Thomas suffered a traumatic brain injury and died in St Vincent's Hospital two days later.

Following his death, Thomas' family, including his parents, Kathy and Ralph Kelly, brought the public's attention to alcohol-related violence in the Cross.

Their fierce advocacy created the momentum that led to strict mandatory sentencing laws being introduced for violent offences fuelled by alcohol.

The NSW government would also tighten its lock-out laws in Kings Cross and other busy city nightspots. It has been credited with contributing to a drastic reduction in alcohol related crime in Kings Cross.

However, the family's place in the spotlight has been taxing.

Earlier this year, death threats were made to the Kellys after reports emerged about the salary Mr Kelly drew from the Thomas Kelly Youth Foundation.

The foundation started the Take Kare Safe Spaces program, which has seen volunteers on Sydney's streets involved in more than 18,000 interventions with young people in need in the past 18 months.

Foundation board member and family friend David Anstee said Stuart had been an absolute supporter of his family's and the foundation's achievements.

"Our hearts go out to the family at this time," he said.

"The work that the foundation has done and his parents have done has been amazing. Stuart was an absolute supporter of this.

"What this family has done to help support and bring our kids home safely is one of the most amazing achievements, beyond the capabilities of most in the circumstances.

"To suffer any more grief is beyond belief."

Mr Anstee said the Kelly family asked for privacy during this difficult time.

Tim Hawkes, headmaster at The King's School, which both Stuart and Thomas attended, released a statement on Monday night expressing his shock.

"To have lost two sons in such circumstances is beyond awful," Dr Hawkes said.

"As a student, Stuart was greatly respected. The strength and character shown by Stuart when his brother was killed ... was extraordinary."

Last year, at a gala event to raise money for the foundation, Stuart addressed an audience of more than 700, including the state's most powerful figures, and revealed the lasting impact his brother's death had on him.

He recalled the moment he and his sister, Madeleine, learnt that Thomas' life support would be switched off.

"I look back at that moment: I was 14 years old, I was told by a stranger that my brother, my best friend, was going to die. Those few words would change my life forever," he told the crowd, including NSW Premier Mike Baird, NSW Police Commissioner Andrew Scipione and lord mayor Clover Moore.

"I'm now 17 - that was three years ago. However, I carry a deep scar that you cannot see. It's always there, it never leaves. It sits below the surface of your skin and surfaces when you least expect it."

Stuart displayed stoicism and composure beyond his years that night as he shared, with a room mostly full of strangers, his last memory of Thomas alive and his journey to his critically injured brother's hospital bedside.

"Tom never deserved to die that night, it was not meant to be his time. In fact, I believe now that it could and should have been avoided. Our family lost a son and a brother," he said.

"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."

He also appealed to Mr Baird and the community to combat Australia's drug and alcohol abuse problem.

Loveridge is serving a minimum of 10 years and two months in jail for Thomas' death and other violent offences he carried out in Kings Cross on that fateful night.

❏ Support is available for those who may be distressed by phoning Lifeline 13 11 14; Mensline 1300 789 978; Kids Helpline 1800 551 800; beyondblue 1300 224 636.