Stuart Kelly, the younger brother of coward punch victim Thomas Kelly, has died.

Police said there were no suspicious circumstances in the death of Stuart, who was 19.

"We look after our own and we support each other when we're hurt. We are in a state of grief at the moment at the loss of Stuart Kelly.," Kings School headmaster Tim Hawkes said in an email about the school's former pupil.

"To have lost two sons in such circumstances is beyond awful."

Stuart graduated from Kings last year.

Thomas Kelly died after he was punched by Kieran Loveridge in July 2012. (AAP)

He had stepped into the spotlight when he gave an emotional speech he made at a gala dinner last year, detailing the moment his family was told life support for Thomas would be switched off.

Thomas was randomly punched by a stranger in Kings Cross in July 2012, knocking him unconscious.

He later died in hospital. He was 18.

His killer Kieran Loveridge was found guilty of manslaughter over the unprovoked attack and is behind bars.

The attack prompted controversial lock-out laws implemented by NSW Premier Mike Baird, aimed at ending alcohol-fuelled violence in Sydney’s CBD and party district.

Stuart was 14 when Thomas died.

NSW Police confirmed a report was being prepared for the Coroner following the death of a 19-year-old in Mona Vale, in Sydney's north.