David Seymour says calls to investigate Hobson's Pledge shows hate speech legislation is "now being used as a political weapon".
The New Zealand Māori Council has asked the Human Rights Commission to investigate the group, led by former ACT and National leader Don Brash, because "no one's called them out".
"They may wear suits and drive around in late-model expensive European cars… but they are nothing more than a gang of misfits that seek to incite hate and divide the country," executive director Matthew Tukaki told Stuff on Wednesday.
"They're creating an environment... in which hate is breeding and not just breeding but duplicating and replicating."
Hobson's Pledge campaigns against what it says is bias and favouritism towards Māori.
"The idea that all New Zealanders should be treated equally before the law is a genuinely-held political view," said Seymour, present leader of ACT and its only MP. "There is nothing inherently racist or violent about it.
"What the New Zealand Māori Council is proposing is that the Human Rights Commission be asked to endorse these kinds of attacks by applying state power to the respondent. Hate speech legislation is now being used as a political weapon."
Seymour said it was ironic Tukaki said Hobson's Pledge would "send us all back to the dark ages".
"Asking a king or tribal leader to settle political scores is exactly what happened in the dark ages."
Prime Minister Jacinda Ardern earlier this week said there was a "gap in our legislation" when it came to hate speech.
"There is a very big difference between criticism and even sharing views that people necessarily wouldn't agree with, there is a very big difference between that and language which is designed to incite harm against others," she told The AM Show.
"That is something that we jealously guard in our legislation and our democracy, but at the same time, of course, people would expect that if you are trying to incite violence and potentially murder against groups of individuals based on their beliefs, no one in New Zealand would think that was okay either, so it is just making sure we get that balance."
- Duncan Garner: Free speech 'vital' in free society
- 'There should be limits' to free speech - Council for Civil Liberties
- Free speech debate reignites after Christchurch terror attack
Seymour says the law shouldn't be tightened, despite growing reports of white supremacist activity online, powered by social media.
"If the Government broadens our hate speech laws, we will see more New Zealanders hauled before a state agency for simply expressing their political views. That really would divide the country."
Dr Brash told Stuff he doesn't drive a "late-model expensive European car" - instead getting behind the wheel of a "four-year-old Korean Hyundai".