Seymour also said Williams had a "choice" and was now facing the consequences of his actions.
"People make choices, you've made your choices," Seymour told him.
"I was expressing my freedom," Williams responded.
"It has consequences, and now you're suffering the consequences of your actions," Seymour told him.
The YouTuber called Seymour a "fake" and "a bloody fraud" and said that "Māori will own 50 per cent of the country by 2040".
The man then turned to the crowd and introduced himself under the name of his YouTube channel, as well as Lee Williams.
"You're an idiot, mate," Seymour responded, while others in the crowd told Williams to go home.
"You're making a d***head of yourself," people in the crowd told Williams, as Seymour walked away from him as he continued to shout.
Williams was fired from his job at Synlait last month after a petition calling on the firm to "stop employing white supremacists" gained more than 7700 signatures.