NZ First is considering laying a formal complaint against Parliament's Speaker, David Carter, claiming he insulted party leader Winston Peters.
The party's deputy leader, Ron Mark, says Mr Carter muttered "Ahh, shut up Winston" during question time yesterday.
It wasn't heard at the time, but Mr Mark says it's "as clear as a bell" on the official video recording of question time.
"It's not just an insult to the party leader, it's demonstrably bad behaviour coming from the man who presides over parliament," Mr Mark told NZ Newswire.
"This is the most appalling behaviour from the Speaker, whose job is to ensure order in parliament."
A spokesman for Mr Carter said the Speaker didn't wish to comment.
In Parliament today Mr Mark tried to table a transcript of the remark, but Assistant Speaker Lindsay Tisch wouldn't allow it.
Mr Tisch told Mr Mark there were other courses he could follow, and he should give notice of his intentions.
Mr Mark says he's taking Mr Tisch's advice and is going to discuss the issue with Mr Peters.
"I'm pretty confident this isn't the end of the matter," he said.
The comment is not in Hansard's account of question time yesterday.