Prime Minister John Key has rejected any suggestion that an inquiry ordered into allegations against former minister Judith Collins is a whitewash.
The Labour Party and New Zealand First say the investigation is too narrowly focused.
Mr Key has appointed former High Court judge Lester Chisholm to conduct the inquiry into allegations that in 2011, as Justice Minister, Judith Collins undermined the then Director of the Serious Fraud Office, Adam Feeley.
Mr Chisholm will look into whether there is any evidence that Ms Collins acted inconsistently with the conduct expected of a minister by undermining or attempting to undermine Mr Feeley, and whether she provided information about him to blogger Cameron Slater or anyone else who was not entitled to get it.
It will also look at whether Ms Collins inappropriately sought or received information about Mr Feeley from Mr Slater or any other party.
The inquiry will identify and report on any other issues relevant to the above matters, to the extent necessary to provide a complete report on those matters.
Mr Key said the inquiry would have the full powers under the Inquiries Act 2013 which has the power to call people to give evidence under oath.
He defended the focus of the inquiry, saying it could only look at Ms Collins' actions, rather than those of Mr Slater and others.
"That is my focus of attention - it's my minister and what engagement she had with her chief executive and whether it was professional. The terms are very broad within that. It's every piece of information relating to that and anything else that Justice Chisholm wants to look at."
Lester Chisholm is a Companion of the New Zealand Order of Merit for his services to the Judiciary and was a member of the New Zealand Law Practitioners' Disciplinary Tribunal. He was also recently the chair of the Board of Inquiry of the Ruataniwha dam and irrigation scheme.
The inquiry will begin on 12 September and report back by 28 November.
Cover-up, says Cunliffe
Labour Party leader David Cunliffe says today he doesn't have confidence in the terms of reference for the inquiry into allegations against Judith Collins.
Mr Cunliffe said the allegations go a lot further than just Ms Collins, and the inquiry is too slow and too narrow.
"We know Cameron Slater's emails involve more than Judith Collins' office. They involve John Key's office, with (former staffer) Jason Ede. They involve (minster) Amy Adams. This inquiry is not broad enough to do the job New Zealanders want done - which is clean up the mess. This is not an inquiry, this is a cover-up."
Meanwhile, New Zealand First leader Winston Peters said the terms of the inquiry set by Mr Key are completely inadequate and will not get to the bottom of anything.
"He should have had the prescience of mind and the understanding of our constitutional democracy to leave this till after the election and have a proper inquiry with appropriate terms of reference to get at the truth."
The general election will be held on 20 September.