A teenager who fatally stabbed another boy in the heart during a brawl on a Sydney train was only acting in self-defence, his lawyer says.

The 16-year-old boy has been committed to stand trial in the NSW Supreme Court for murder and affray after he admitted stabbing the 17-year-old in the chest, fatally penetrating his heart.

The stabbing occurred during a fight between two groups of teenagers on a train at Campsie railway station in the city's south on December 21 last year.

But the accused says he was acting in self-defence after the older boy entered the train swinging his fists.

"It's going to be conceded that there's sufficient evidence for the young person (the accused) to be committed for an indictable offence, but there's possibly an issue as to whether or not there's a genuine issue of self-defence," the boy's lawyer Dennis Miralis told a committal hearing at Parramatta Children's Court on Thursday.

"There's no issue that he stabbed the deceased ... (but prior to that) the young person was subjected to an attack by the victim in the company of a number of males."

The younger boy suffered contusions, scratches, cuts, bruises and a suspected fractured nose in the melee, his lawyer said.

CCTV footage of the brawl, which lasted a few seconds, was played to the court, and prosecutors conceded that the elder boy threw the first punch.

"As the victim enters the (train) compartment he appears to have a raised fist, or his arm was raised in some way and starts to strike," Martin Jilek said for the Director of Public Prosecutions (DPP).

"The young person (the accused) is seen to be removing his hand from his pocket, and the prosecution says that is when he produced the knife.

"The prosecution will concede that the victim struck the defendant first."

Mr Jilek denied the accused boy was acting solely in self-defence, adding that his preparation for the fight was evident in the fact he was seen producing a knife from his right-hand pocket.

"(The stabbing) was all in one movement and it's almost a pre-emptive strike, rather than a defensive strike," he said.

Another boy told the court the 17-year-old boy had wanted to follow the other group because they had "had an argument with my friend".

"I thought he was going to say a couple of words back and forth, but I wasn't expecting a fight," the teenager, who cannot be named, said.

"When it happened, it happened ... once it broke out we all ran in."

Mr Miralis submitted the judge should consider committing the younger boy to stand trial for manslaughter, instead of murder, on the basis he used "excessive force" in self-defence.

But Judge Mark Marien disagreed, saying the boy was in company and could have asked his friends for help if he felt he was in trouble.

Being the victim of aggression did not give him the right to pull out a weapon, the judge added.

"He struck him with a knife ... he actually succeeded in puncturing him in his chest twice," he said.

"Did he really believe that that response was necessary to defend himself when the victim was unarmed? It was a fist fight."

The boy's bail has been continued and the matter will be mentioned in the NSW Supreme Court on December 4.