Labour Party leader David Cunliffe says if he was Prime Minister he would hold his ministers to a higher standard than John Key does.
Mr Key is under pressure to fire Justice Minister Judith Collins, who gave the name of a public servant to blogger Cameron Slater.
The minister suspected the public servant, Simon Pleasants, had leaked information about Finance Minister Bill English double-dipping on his accommodation allowance. Mr Pleasants had not leaked any information, but was subject to abuse on Mr Slater's Whale Oil blogsite and also received death threats.
Opposition parties and the Public Service Association say Ms Collins should go but Mr Key is standing by her.
David Cunliffe told Radio New Zealand's Morning Report programme Ms Collins' behaviour was unbecoming of a minister, especially a Justice Minister who is meant to uphold and guide the formation of the law.
He said if Ms Collins was a minister in a government run by him, she would be gone.
"There would be a huge difference in the standards that I would hold my ministers accountable to and the way that the Prime Minister is treating the Minister of Justice."
Mr Key not see anything wrong in what Ms Collins did and yesterday expressed no regret for the threats Mr Pleasants faced as a consequence.
"I asked her this morning specifically and she told me she had passed on the title, that he already knew the name but again she's no shrinking violet. You guys are free to interview her anytime you want," he said.
Mr Key said those on the left also talk to bloggers and give them information, but David Cunliffe told the programme there was no comparison.
"I think Mr Key is flailing around making excuses for a machinery that he, his office and his supporters have built up which is involved in the systematic smearing and denigration of individuals including public servants.
"I know nothing on the left of politics that is equivalent to what is being exposed in Mr Hager's book.
"I've appeared above board in interviews on several left-wing blogs, there's nothing wrong with that. And from time to time staff provide information to blogs as they do to press gallery journalists. There's nothing wrong or unusual with that.
"The issue here is the kind of material being pumped out through the Whale Oil blog is completely below the belt."
Collins should go, says union
Public Service Association acting national secretary Glenn Barclay has repeated his call for Ms Collins to resign and said it was disturbing the Prime Minister would not investigate her actions.
"It is concerning that the Prime Minister has batted this away, saying he just fully stands by the minister (and) is not even prepared to investigate it," he told Radio New Zealand's Nine to Noon programme.
"In light of the fact that she's admitted leaking the information, at the very least an investigation would be called for, but we think that her resignation is probably appropriate."