The parents of an 18-year-old who died after being king-hit in Kings Cross at the weekend have spoken of their anguish at their son's death.

Thomas Kelly was attacked as he walked along Victoria Street with two friends just after 10:00pm on Saturday.

An unknown man came towards them and punched Thomas in the face, knocking him back onto the pavement, before fleeing the scene.

Kings Cross LAC crime manager Detective Inspector Paul McDonald says the brutal attack was unprovoked.

"This was a cruel and senseless attack on Thomas, who was visiting the Kings Cross area for the first time to celebrate a mate's birthday," he said.

"Thomas was simply minding his own business, walking along Victoria Street with friends, when he was punched to the head, causing him to fall to the ground."

His parents were called to St Vincent's Hospital where Thomas was treated for serious head injuries and his life support was switched off on Monday.

"We went back down to the hospital at midnight to give a final identification check and to say goodbye, and it is the hardest thing that any father or mother can ever possibly do," his father Ralph Kelly said.

Mr Kelly said he and his wife had urged Thomas to be careful in Kings Cross.

"We didn't want Thomas to go to Kings Cross that night, but you can't tell an 18-year-old boy what he can do," he said.

"And the last words that both Kathy and I said to him was 'Please be careful, please take care'. He said 'Don't worry, I will and I love you'. That was the last time that we spoke to him."

Mr Kelly has appealed for anyone who has information on the attack to come forward.

"This is just a tragic, tragic situation. Nobody would want to be in this position," he said.

"We are sitting here today, on behalf of our son, to appeal to the public to please come forward with any information that they can give to the Kings Cross detectives or the homicide department to help us bring closure to this for us."

'Beautiful kid'

Thomas Kelly was attacked as he walked along Victoria Street with two friends on Saturday. ( Supplied )

Struggling to maintain composure, Kathy Kelly echoed her husband's plea for information.

"What it has done to our children and what this has done to us, we don't want it to happen to anybody else," she said.

"So if anybody can come forward and say something to get this person off the street so it doesn't happen to another family and nobody else is ever in the position we're in."

She said her son's life had been cruelly cut short just as he was beginning to find his purpose in life.

"I don't know what else I can say, except he was just the most beautiful kid with his whole life in front of him. And it has taken him 18 years to feel comfortable and loved, and he's finally got his feet on the ground and had a purpose in life, and it is gone," she said.

Police have formed Strike Force Mohawk in response to the incident, and Detective Inspector McDonald says witnesses are needed to bring the attacker to justice.

"Thomas's injuries were so severe that he would not recover. What should've been a fun night out on the town with his mates ended in the tragic loss of a young life and absolute heartbreak for his loved ones," he said.

"I would urge anyone who witnessed this callous assault on an innocent young man to come forward and help us bring the offender to justice."