"We had a difference of views about the way I was refereeing and the way he was coaching," Hallahan said. "Reynolds was right in my face and I was telling him to go away, when 'bang' - he suddenly just nutted me.
"I was stunned and fortunately I didn't react. I was aware there were 6-year-old boys on the field and I didn't want them to see any more violence.
"To do something like that in front of his son and mine was despicable and cowardly. I just asked him 'what the hell did you do that for?"'
The Helensville club called an emergency disciplinary meeting the next day. As a result, Reynolds was stripped of his coaching role and banned indefinitely from attending matches.
The club's junior chairman, Nick Mulvaney, said there was no option but to come down hard. "His behaviour was completely out of character. But we have codes in place and it was decided Bryce should stand down and also be banned from the ground," Mulvaney said.
"Bryce understands and accepts the situation. He is upset he will no longer be able to come and watch his son playing rugby, but we just can't have things like this happening at a kids' game."
Hallahan said Reynolds emailed and texted the team to apologise.
"The guy wasn't even man enough to apologise to me in person and that just angers me even more," he said. "I am from Ireland and was proud my son has been learning to play in New Zealand. What kind of example is that for young children?"
Hallahan said he had to take time off work because of his injuries and had reported Reynolds to police. Hallahan added his doctor had advised he will require surgery to fix his nose.
Reynolds could not be reached for comment. But his tearful wife Nicola confirmed he had been banned from the Helensville ground.
Helensville police said a complaint had been laid and they were investigating.