More than 35 police called in to tackle a crowd of angry protesters outside the Labour Party's annual conference in Auckland.
The violence erupted when Labour Party member Len Richards went into the crowd and allegedly hit a protestor following a struggle with the megaphone he was using.
Richards was not arrested but several protestors were taken away by police.
The relatively peaceful protest, which involved around 80 people, was staged in opposition to the Terrorism Suppression Act employed in recent police raids.
A man who tried to intervene was dragged away and taken to a police van where he was cuffed and searched.
The mood of the protest continued downhill when a young Maori protester doing the haka confronted a police officer and spat at him. Police took the 17-year-old away. He was charged with technical assault but was later released.
Protesters are furious their people are being arrested while those that started it go free, but Richards has repeatedly said he has done nothing wrong.
"It didn't hit anybody. If I did it was because I felt threatened, but I didn't go out there to hit anybody. There was no violence...I oppose violence," he says.
Police spoke with Richards but showed no interest in arresting him.
A protester is understood to have laid a complaint against Richards and police say they are investigating.
