Coward punch victim Thomas Kelly's brother has died.

The brother of “one punch” victim Thomas Kelly, Stuart Kelly, has tragically passed away.

Stuart, 19, died under unknown circumstances at his home in Mona Vale on Monday.

He was only 14 when his older brother Thomas was killed by a coward’s punch in King’s Cross four years ago.

The King's School, where Stuart was an Old Boy, released a statement last night informing the school of Stuart's death.

“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,” the headmaster said in the statement.

“The exact circumstances surrounding his death are not known, however, it is enough to know that we have lost a member of our community and therefore our thoughts and prayers go out to Kathy and Ralph Kelly and their daughter Madeleine.”

Stuart had previously spoken of his heartbreak over his brother's death.

"I carry a deep scar that you cannot see," he said in an address at a gala for the Thomas Kelly foundation 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 believe now that it could and should have been avoided. Our family lost a son and a brother.

"I ask all of you to look at me, I am but one person who has been affected by violence.

"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."

Watch Stuart Kelly speaking on The Project about the tragic loss of his big brother.

David Anstee, a board member of The Thomas Kelly Foundation, has extended his sympathy to the Kelly family following the tragic news.

"Our hearts go out to the family at this time," he told The Sydney Morning Herald. 

Stuart Kelly was a vocal spokesperson for the Thomas Kelly Foundation, which was established to prevent further deaths from one punch attacks.

"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."

On July 7, 2012, Thomas Kelly was walking with his girlfriend in King's Cross when he was brutally attacked by Kieran Loveridge. Two days later, he died in hospital from a traumatic brain injury.

Loveridge was originally sentenced to a minimum jail term of five years and two months, but this was later increased to a maximum sentence of 13 years and eight months.

NSW Police have advised that the circumstances surrounding Stuart's death are not suspicious, and that the Coroner will prepare a report.

Feature image via Youtube/The Project. 

If you or a loved one is battling depression or suicidal thoughts, Mamamia urges you to contact LifeLine on 13 11 14, or visit their website.