Labour Party leader Phil Goff and the Security Intelligence Service disagree over whether he was briefed about an investigation into whether Israeli backpackers caught up in the Christchurch earthquake were Mossad agents.
Mr Goff says he was never shown a document by the SIS about the so-called Israeli spy investigation.
But SIS director Warren Tucker has released a note under the Official Information Act to blogger Cameron Slater which includes a notation that Dr Tucker believes he briefed Mr Goff on the matter.
Mr Goff says he will not have his credibility questioned.
He says in the future he will only meet Dr Tucker or any other member of the SIS if someone independent is in the room to keep a true and accurate record of what is discussed.
But Deputy Prime Minister Bill English on Thursday criticised Mr Goff's attack on Dr Tucker.
Mr English says he knows who he believes. "If you were given the choice, I think a senior civil servant who has advised the previous prime minister and the current prime minister on issues of national security - a man of integrity - I'd believe him."