Stuart Kelly took his own life after receiving relentless hate mail over the Sydney lockout laws which were triggered by the coward punch death of his brother Thomas, News Corp reports.

Thomas Kelly died in hospital in 2012, two days after he was randomly struck in the head by Kieran Loveridge while on a night out in Kings Cross.

Stuart, 19, described the loss of his sibling as a "deep scar" that he always carried with him.

He was found on Sydney’s northern beaches on Monday after taking his life.

Thomas, Ralph and Stuart Kelly. (Source: Thomas Kelly Foundation)

"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," Stuart said during a gala dinner for the Thomas Kelly Foundation in September last year.

"It’s always there, it never leaves. It sits below the surface of your skin and surfaces when you least expect it.

"Tom never deserved to die that night, it was not meant to be his time; in fact I now believe that it could and should have been avoided."

As a result of Thomas’ death the NSW state government imposed strict lockout laws for most of the CBD which have reportedly led to the closure of several bars and nightclubs.

Stuart and Thomas Kelly. (Source: Youtube)

A close friend of the Kelly family told News Corp in the years since his brother’s tragic death Stuart was subject to a torrent of abusive hate mail and online bullying.

Despite graduating from The King’s School and being accepted into the University of Sydney to study marine biology, he reportedly deferred his studies and returned to his high school to coach an under 14 rugby team.

The King’s School principal, Dr Tim Hawkes has spoken spoke highly of the younger Mr Kelly.

"The strength and character shown by Stuart when his brother was killed by a one punch attack in July 2012, was extraordinary," Dr Hawkes said in a statement.

"The death of Stuart Kelly is tragic, and a reminder that life is fragile, loved ones precious and each day a gift."