The parents of Thomas Kelly, the 18-year-old who was fatally king-hit during a night out in Sydney's Kings Cross last year, have described their son's killer as a coward.

Ralph and Kathy Kelly stood up in the New South Wales Supreme Court this morning to read victim impact statements at a sentencing hearing for Kieran Loveridge.

Loveridge pleaded guilty to manslaughter in June, after the charge was downgraded from murder.

He randomly punched Mr Kelly in the head in July 2012, knocking his victim to the ground and causing catastrophic brain injuries.

The 18-year-old was taken to St Vincent's Hospital, but his life support was switched off two days later.

Today Ralph Kelly told the court he would do anything to swap places with his son.

"He never should have died from such a meaningless act of cowardice," Mr Kelly said.

Mrs Kelly told the hearing she thinks of her son endlessly and cannot understand why he is gone.

Loveridge will be sentenced at a later date for manslaughter and several other assaults he committed on the same night.

When Loveridge pleaded guilty in June, Mrs Kelly said her family had no rights in the decision to downgrade the charge, and warned that violent offenders will "remorselessly reoffend over and over again".