Asked what contact he had with bloggers, Mr Cunliffe said it was "very, very rarely that I would speak to a blogger, very rare indeed".
The public still needed answers over allegations in Hager's book, including the speed in which Slater had received a response to an Official Information Act request from the Security Intelligence Service, he added.
The release of the documents, involving a briefing to former Labour leader Phil Goff by SIS boss Warren Tucker on Israeli agents in Christchurch, were alleged to have been received on the day of request and were used by Slater's Whale Oil blog to embarrass Mr Goff.
Mr Key, the minister responsible for the SIS, yesterday denied giving approval to the release of the documents to Slater, adding: "This is an issue from three years ago that most voters aren't that interested in talking about."
Processes had since improved, he said.
Mr Cunliffe said the Minister for SIS was an "important and sensitive role and should always know when matters like that - this one involving the leader of the Opposition - are being signed off - if he didn't do it, who did?"
Mr Cunliffe said Mr Key also needed to relieve Justice Minister Judith Collins, after she confirmed providing Slater with the name of a civil servant alleged to have leaked details about ministerial spending to Labour.
"I don't know why the Prime Minister is not addressing the issue with that minister - he put her on her last warning months ago, and there's ample evidence in the book that she's unfit to be any minister - let alone Minister of Justice."
Mr Key again dismissed the book yesterday. "People can see that Nicky Hager's made a whole lot of things up in his book. They can see that he can't back a lot of them up."