The blow landed him in hospital for three days and caused him permanent damage with doctors advising him not to play contact sport in future.
His parents - who spoke on condition of anonymity - said their son had been left with three plates in his jaw, numbing down the right side of his face and would require serious dental work in future.
Initially they voiced their disgust about the potential for Birtwistle to escape conviction but after a restorative-justice conference, were able to give him their blessing.
At the end of last year, restorative justice was made mandatory in cases where there was an identifiable victim but the legislation change resulted in a backlash from some lawyers who saw it as clogging up the court system.
Judge Thorburn held this case up as an example of how successful the scheme could be.
"The belief that is growing is that these processes, if successful, have got much more capacity to put people in real touch with the consequences of their offending than the clinical and mechanical ways of court processes and sentencings," he said.
"This is what happened here."
The judge granted the discharge in March but made it conditional on the defendant completing an anti-violence course and writing a letter to the victims telling them of his process.
It is understood there was also a confidential payment made to the victim.
"He is a young man with the expectations of a professional life ahead of him. He has possibilities of some connection with a sporting career," Judge Thorburn said.
"He has got... the impeccable character which it would be a major consequence to contaminate because of one idiotic moment of one's life."
However, he pointed out how different Birtwistle's plight could have been, had the incident happened in Australia, where there are long jail terms in some states for perpetrators of "king-hits".
"Perhaps in a counting of blessings Mr Birtwistle is fortunate that he is not across the other side of the Tasman," he said.
The judge hoped the incident had been a "massive trigger" in developing empathy in the defendant, which he said arrived "a little later for the male of our species than females".
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