Andrew Little is accusing Winston Peters of leaking poll results that are damaging to the Labour Party.
Newshub revealed last week results from polling company UMR had been leaked, and the Labour Party leader says he has an inkling of who's behind it.
"Whenever you see something that's reported as a leak, you look at who talks about it the most," he told Newshub.
"I'm pretty sure NZ first has UMR as a pollster, so I think the leak - in inverted commas - is more likely from New Zealand first than anybody."
The results which show Labour on 26 percent, down from 34 percent in May.
But Mr Little says he's not particularly fussed, saying polls "bounce around".
"One thing you can say about Winston Peters is he's one of NZ's more colourful characters - he has principles now and then, but he is a blowhard."
Mr Peters says he doesn't care what Mr Little thinks.
"I've got no reaction to that. I couldn't give a rat's derriere what he says," the NZ First leader told The AM Show on Monday. He wouldn't reveal if NZ First used UMR for its polling.
"We don't divulge who we talk to on the issue of polling… That's not information you're privy to... It's none of your business."
Mr Peters has been talking up the poll regardless, suggesting he'll soon have the right to call himself leader of the Opposition.
"If [Labour] go from 26 down to 22, that's it. Andrew is not in Parliament," Mr Peters told The Nation on Saturday. "So why would you make these statements, that he's the next leader of the country? Or the leader of the Opposition?"
The leaked results showed New Zealand First on 14 percent, overtaking the Greens on 13 percent.
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If Labour drops to 22 percent, Mr Peters is right - Mr Little likely won't make it back into Parliament, because Labour's quota of MPs will be filled by its probably electorate winners. Mr Little isn't contesting an electorate.
If Labour are to form the next Government, they will likely need NZ First and the Greens onside.