Prime Minister Jacinda Ardern "categorically" ruled out that Simon Bridges' expenses leak came from Labour, and Mr Bridges said he was "confident" it did not come from National.
Mr Bridges' MP expenses were released yesterday, with $113,973 spent on travel and accommodation between April and June, reported by Newshub.
Ms Ardern said Labour did not have access to Mr Bridges' expenses.
"We've sought assurances from Ministerial services who manage this information that none of the Opposition's numbers were shared with anyone but them," Ms Ardern said. "They've given us that assurance."
"We've had it confirmed no one in Labour actually ever had access to that information. The only groups as I understand who will of had access will be the Opposition themselves and the Speaker."
She said even if Labour had access to it, it would be "totally inappropriate" to leak it.
Mr Bridges said he did not know who leaked the details of his expenses.
"It isn't National, I'm very confident about that. It's good to see [Speaker] Trevor Mallard is doing an inquiry and we'll see what happens.
He said there were "multiple players throughout the parliamentary precint" that could have had access to the figures.
"The reality is this was going to come out on Thursday, it's not that big of a deal."
Mr Bridges did not think it was an overspend, and that it was "fundamentally important" that he got out around the country.
Speaker Trevor Mallard said the leaked document had a different format than that supplied to Parliamentary Services, Parliamentary Library and from the Speakers' office.
He said he would take 24 hours to "take soundings about whether there needs to be a further forensic examination as to the source of the document".
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