The Serious Fraud Office rejects Prime Minister Helen Clark's accusation it leaked information to the National Party.
Clark accused the SFO of tipping off the National Party about its decision to investigate donations to New Zealand First.
She said on Monday it was "almost certain" National knew the announcement was going to be made last Thursday, the day after party leader John Key said he would not work with NZ First leader Winston Peters after the election.
"I find the National Party's statement and timing interesting because I would say it's almost certain they got a tip from the Serious Fraud Office that it was about to move," Miss Clark said on TV One's Breakfast programme.
A spokeswoman for the SFO said that checks had been done and Clark was wrong.
"We do not believe that there has been any leak from the SFO and have received assurances to that end from all SFO personnel involved with the donations matter."
The office would make no further comment.
National leader John Key also denied any leak.
"National simply didn't know about the SFO's intentions and Helen Clark should apologise to the agency for her false claim," he said.
Key said he would not work with Peters under any circumstances, significantly hardening his stance and changing the political landscape under MMP.
Both the main parties have in the past refused to rule out working with any of the minor parties after an election, because they might need them to form a government with a majority of votes in Parliament.
"We have categorically ruled him out altogether," Key said on Sunday when he was asked whether he would work with Peters.
Key's decision has put the Maori Party into a potentially strong position after the election.
It is expected to increase its number of MPs - it has four at present - and could hold the balance of power if National doesn't win an outright majority.
The fracas over donations has also prompted Grey Power to issue a statement saying it does not have any political affiliation.
Grey Power NZ Federation national president Les Howard said the organisation was not responsible for politicians saying they had its support.
He said members were of all political persuasions. The organisation advises members to vote how they wanted for the electorate but reserve their party vote for whatever party's policies were closest to Grey Power policies.