"There was violence and a confrontation. This was parents behaving badly."
Mr Ammunson said the brawl did not involve any players, team coaches or match officials, who abandoned the game.
"There were no coaches, officials or children involved in the incident - as soon as it erupted, the two coaches pulled their players aside. But we are really disappointed the children have been exposed to something like this."
Mr Ammunson said there were union and club policies regarding sideline behaviour and he expected sanctions to be imposed that could include bans.
"Both clubs have given assurances there will be implications for the people involved," he said.
"I would expect that any spectator who becomes violent at a rugby ground would be facing a ban as a possible consequence. The individual clubs will need to make those decisions."
Mr Ammunson was aware of a similar incident at an under-10 match last year between Martinborough and Pioneer that he said was unrelated.
Last night he was to discuss the incident and the union's constitutional powers and obligations with chief executive Tony Hargood and Junior Rugby chairman Doug Bracewell, he said.
"The most important thing is that families and children feel comfortable and safe and have a great time at sport.
"This is a tragedy, really, that children were exposed to this, and we don't want it to happen ever again."
Mr Ammunson said board members last week talked about holding a review of "club culture, and what we can do to help set a high standard for clubs to be safe, fun and comfortable".
He said the call for a review was prompted in the wake of an incident involving Wairarapa rugby star and Pioneer Rugby Club player Sean Robinson, 19, who was left with a serious head injury after an alleged late-night assault at the hands of a clubmate.
"The incident with Sean made our review a matter of priority. We want clubs to be places with a good and positive culture, not a place where these kinds of things happen.
"I want a full and broader review of sideline behaviour and club culture, and to have a look at new policies and what we can do to improve things," Mr Ammunson said.
"We'll learn from this, the clubs will learn from this, and it will prompt us to work even harder to ensure our club grounds are safer."