"This time he's not going to get away with a rugby analogy and a smile.
"The better analogy is if someone's house is unlocked and a burglar comes and steals the purse off the table, that's still theft, even though the house wasn't locked."
Asked what contact he himself had with bloggers, Mr Cunliffe said it was "very, very rarely that I would speak to a blogger, very rare indeed, and I certainly don't speak to anyone of the nature of [WhaleOil blogger Cameron] Slater.
Mr Cunliffe said the public still needed answers over allegations in Mr Hager's book, including the speed in which Mr Slater had received a response to an Official Information Act request from the Security Intelligence Service.
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 Mr Slater's WhaleOil blog to embarrass Mr Goff.
Mr Key, the minister responsible for the SIS, this morning denied giving approval to the release of the documents to Mr 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?"
"This has never been standard process for any minister, let alone the minister for the SIS. Mr Goff has informed that there are some very unusual circumstances about the nature of that."
Mr Cunliffe said Mr Key also needed to relieve Justice Minister Judith Collins, after she confirmed providing Mr 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 this morning refused to answer questions from Radio NZ over whether this was "okay" and said he would not be raising the issue with Ms Collins.
Mr Key again dismissed the book and questioned its credibility.
"At the end of the day we're five weeks out from an election. People can see that Nicky Hager's made a whole lot of things up in his book. He can see that he can't back a lot of them up."
Asked whether he would continue pressing for questions raised by Mr Hager's book throughout the election campaign, Mr Cunliffe his team was "dealing with the detail of that" but he was personally focussed on running a "positive campaign".
• Check out the full coverage of Hager's book here.