The summary said: "The defendant was sick and tired of the victim's behaviour towards his sister; he decided enough was enough.
"He removed his belt and struck the victim around the legs once and then on his shoulders once with the leather tongue section of the belt."
The boy received minor bruising to his shoulder and redness and bruising to his lower leg. He received medical attention for his injuries.
It is believed the boy's school alerted authorities including Child Youth and Family and the police.
The school is understood to have wanted the boy to receive medical treatment to ensure he did not have any long-term damage.
When the the former player, who is in his 40s, was spoken to by officers he explained he was at the "end of his tether" over his children fighting.
Yesterday, he said he could not comment while the case was before the court because to do so would jeopardise proceedings.
Until now, only bare details of the charge have been made public although the case was always considered to be one of excessive discipline than a more serious assault.
The incident happened at the family's Auckland home on the afternoon of September 27.
When the former rugby star appeared in Auckland District Court last month he was given interim name suppression, prompting criticism that it was another case of celebrities getting preferential treatment from the justice system.
Judge Tony Fitzgerald said interim name suppression was "primarily for the protection of the victim, given his age, and the stress that it would likely cause for your name to be published, given your profile in the community".
At that appearance, the former All Black pleaded guilty to a single charge of assaulting a child.
The judge told him it was possible he would receive a discharge without conviction if he continued with anger management counselling and committed no further offences.
His sentencing was put off until February, in part so he could continue anger management courses.
Auckland University associate law professor Scott Optican said if the player was given a discharge without conviction he might also get permanent name suppression to prevent the consequences of the offending being disproportionate.
But he felt that would mean the player was escaping without penalty.
"That's about as much as you're entitled to in terms of not having to face the consequences of your actions because it would be disproportionate to the harm. You shouldn't get name suppression on top of that," he said.
News media including the Herald are set to oppose permanent name suppression if it is sought.
