"In 20-odd years of watching basketball I've never seen anything like it. It's a basketball game, it's middle of the season, there's nothing on it.
It was quite bizarre. Some people just need to chill out.
"The girls just stood there stunned," he said.
About 50 people were watching the game but up to 100 were filling the Auckland Boys Grammar gym stands for the premier boys match afterwards, he said.
He said the incident would have been reported to College Sport and possibly Basketball New Zealand because of its severity.
"The kids range in age from about 14 to 18 so they're not young young, but they don't need to witness that.
"The coaches need to, as leaders, show the players the way to react and behave," the spectator said.
Auckland Girls Grammar principal Liz Thomson said the school was aware of the incident but was waiting on a report to confirm the details before commenting further.
She had heard "anecdotal" stories about the incident but could not comment on them.The coach was a volunteer at the school, she said.
College Sport said it could not comment at the time of publication.
Multiple attempts to contact Basketball New Zealand went unanswered.