The 16-year-old's older brother arrived to pick him up from training and saw him on the field with Stephen.
"Acting to protect his brother [the 18-year-old] assaulted Stephen, punching him heavily to the side of his neck," Coroner Gordon Matenga said in his inquest findings, released today.
"[The brothers] assaulted Stephen by punching him several times to his body.
The brothers then left the field and Stephen collapsed and became unresponsive.
He went into cardiac arrest but, tragically, his teammates did not realise the seriousness of the situation and there was a delay in starting CPR; Stephen lay unaided for seven minutes and 30 seconds before someone stepped in.
When paramedics arrived they continued CPR and they managed to revive Stephen.
He went into cardiac arrest a second time, was resuscitated again and loaded into the ambulance.
Resuscitation attempts continued on the way to the hospital, but Stephen could not be saved.
He was declared dead at 6.43pm.
Matenga formally ruled his death was "as a result of cardiac arrhythmia due to stress associated with a physical assault".
Stephen Eruwera Dudley - an amazing young man
On March 20, 2013 Stephen turned 15.
He had his whole life ahead of him.
But 78 days after he marked that milestone birthday, the teenager would be dead.
Stephen was one of five children born to Brent and Mona Dudley.
He is survived by his parents and siblings Talita, Lisa, Aaron and Leith.
The inquest heard that Stephen was in Year 11 at a West Auckland high school - the Coroner suppressed its name - and was a much-loved family member and popular teenager among his peers.
"Stephen was described by his father as an amazing young man who had a really good work ethic, was supportive of his family and of others less fortunate than him and very respectful," Matenga said.
"He enjoyed school, was heavily involved in sports, in particular rugby and basketball.
"In 2013 Stephen had to choose between the two sports as the timings clashed. He chose rugby but still attended school basketball games and training sessions in his spare time."
"I was worried about my brother because he doesn't know how to fight and I was scared he would get hurt.
"I wanted to defend my little brother. I didn't think about much else as I got to Stephen.
"I punched him with my right fist. I don't remember much else of the fight as it happened so quickly.
"I have never thrown a punch in anger before and didn't realise how hard I hit Stephen. He fell to the ground. I thought I had knocked him out and I got scared."
Matenga heard from a number of Stephen's teammates who witnessed the violent attack.
One boy said Stephen was hit with "heaps of force", and that the 15-year-old did not punch back "at any stage".
Matenga ruled there was no fight; that Stephen was the victim of a physical assault by the brothers and died as a direct result.
After Stephen's death, police charged both brothers with manslaughter.
Their cases were heard in the High Court at Auckland.
After a post-mortem examination revealed Stephen had an undiagnosed heart condition that could have been a factor in his death, police withdrew the manslaughter charge.
In March 2014 the younger of the brothers pleaded guilty to assaulting Stephen.
In August 2014 the older boy admitted a charge of assault with intent to injure.
Both boys were discharged without conviction and their names were permanently suppressed.
The underlying heart condition
A post-mortem examination revealed Stephen had an undiagnosed heart problem and that it was "highly likely that the stress of the assault induced a fatal arrhythmia in this young man".
Forensic pathologist Dr Paul Morrow said "everything seemed to be essentially fine until the event occurred and the collapse followed immediately upon that event".
"In my mind, as a medical examiner, as a forensic pathologist, that event was the trigger for his death," Morrow said.
After hearing from Morrow and other cardiac experts Matenga ruled that Stephen's death was not caused by the heart condition.
"I am persuaded by the evidence of Dr Morrow that there is an association in time between the cardiac arrhythmia and the assault," the coroner said.
"That is not to say that other factors did not contribute to the stress which was the catalyst.
"Dr Morrow could not disentangle the contributory role emotional or psychological stress factors may have played, but the main factor, he opined, was the physical assault.
"If the [undiagnosed condition] was the underlying cause why did it not cause an arrhythmia earlier that day, or during the pre-training argument between [the 16-year-old] and Stephen where there was emotional and psychological stress; or during training with the physical stress that placed on Stephen?
"The assault, consisting of heavy blow to the neck and then a series of punches to the torso, must have been the factor which precipitated the arrhythmia.
Matenga said the assault preceded Stephen's collapse and was "the most significant factor which lead to the arrhythmia".
Matenga's formal inquest findings included a number of recommendations.
The first was that Stephen's school, which cannot be named for legal reasons, develop a programme for Year 9 students to learn how to perform CPR and use an automated external defibrillator (AED).
He suggested the training be incorporated into the Year 9 camp programme, which already included basic first aid training, and that it be reinforced every year.
Stephen's parents had asked the Coroner to recommend that all secondary schools encourage students to learn CPR and be confident in giving it and using AEDs.
But he said he believed his recommendation would go further to "reduce the chances of death due to sudden cardiac arrest occurring in the future".
"Students would be given training on what to look for when determining whether CPR was required, how to administer effective CPR and how to use an AED," he said.
"I am satisfied that these actions, as has been seen overseas, will save lives in the future."
Matenga also recommended that the Ministry of Education give all New Zealand schools an overview of the various issues that should be considered when purchasing an AED, including training for staff and students.
The ministry, given the chance to respond before Matenga's recommendations were released, said state schools were Crown entities, each governed by its own Board of Trustees.
As such, although the ministry could provide guidance and support, decisions about teaching and learning were made by individual principals and boards.
Schools also had to consult with their communities before changing or adding to health education, such as CPR training.
The ministry said it did not usually develop guidelines on single issues such as buying defibrillators, but it could include guidance in its health and safety information.
"Such guidance could include material about ensuring staff and students known where an AED is located and when and how to use it," a ministry spokesperson said.
"Given your recommendations, we will prioritise this work."
Matenga said he also investigated other issues arising from the inquest but would not make further recommendations.
The issues included:
• Sports teams training outside school grounds as Stephen's had been. • Coaching and supervision - as the adults had left the training session when the assault happened. • Whether there was a culture of violence within Stephen's school, as his father Brent alleged during the inquest.
The Coroner said he was satisfied that the school's policy around sport's training, whether held on school grounds or elsewhere, was "appropriate and consistent with Ministry of Education guidelines".
"I accept that altering these policies would not have prevented nor reduced the likelihood of death occurring in circumstances similar to the death of Stephen," he said.
In regards to coaching staff at sports practices, Matenga said Stephen's coach "arrived on time, conducted the training and then dismissed participants".
He accepted the coach's evidence that there "was no fighting at training nor was there any sign that day that there were any issues within the team".
"I am satisfied that [the team members] were aware of the strict policy of zero tolerance towards violence at the school.
"No further comment will be made regarding coaching or supervision," Matenga said.
He also said the evidence did not support Brent Dudley's claim of a culture of violence at the school.
Brent and Mona Dudley also asked Matenga to recommend their son's school develop and run a non-violence education pilot programme, "drawing on the painful lessons from Stephen's death".
They wanted the programme to run in addition to the school's zero tolerance to violence policy to "ensure that young people are at least more aware of the risks and consequences of fighting and other violent behaviour".
Matenga heard from the school in response to the suggestion, and was satisfied that it already had "suitable anti-violence, anti-bullying, personal development and guidance programmes in place" being run by "competent professionals".
"There is no evidence to support the suggestion that further programmes are likely to reduce the changes of further deaths occurring in similar circumstances," he ruled.