Deputy Prime Minister Winston Peters says NZ First turned down a proposal from Advance New Zealand to join forces.
"Advance New Zealand wanted to join New Zealand First but we said no," Peters said.
He was speaking in Tauranga's Red Square and responding to a woman holding a poster with images of Prime Minister Jacinda Ardern and communist symbols.
Asked why the party refused, Peters said the reasons would be "obvious to any sensible, sane, rational person".
"Advance New Zealand came and tried to join," Peters said. "We said no."
He said it happened "before they got underway, they had this group before".
But he would not reveal who from Advance NZ approached New Zealand First.
"I won't tell you that. I said I would keep it in confidence."
Advance NZ is co-led by former National MP Jami-Lee Ross and former NZ Public Party leader Billy Te Kahika. Advance NZ merged with the NZ Public Party in July.