The Chief Ombudsman says a major review of the Official Information Act will scrutinise all 27 government ministers' offices.
Dame Beverley Wakem today began her review of the way the public sector used the OIA, which she first announced in August.
Twelve government agencies have been selected for formal review, based on their size, number of OIA requests, complaints, and other criteria.
A further 63 agencies and all ministers' offices have been asked to complete a detailed survey. At least one agency cited for good OIA practice would be included in the review.
The Ombudsman's office would also seek input from past and present public servants, Opposition parties, journalists, academics and others.
Dame Beverley said the goal was to assess the quality and integrity of OIA practice in the public sector and to address any issues that were found.
"The effective operation of the OIA is crucial to our system of open and democratic government, and this review will scrutinise how things are currently operating and set out a framework for systemic improvement where deficiencies are identified."
One of the issues the Ombudsman would investigate was government departments seeking approval from their minister to release information when there was no reason to do so.
Dame Beverley also pointed to recent allegations that OIA processes had been circumvented, "at least one of which has been substantiated in the findings of an independent inquiry".
She did not refer to any specific allegations, but said: "This has the potential to erode public confidence in the OIA throughout the core public sector."
One of the three inquiries prompted by Nicky Hager's book Dirty Politics found that the Prime Minister's office passed on information provided by a spy agency to a blogger for political purposes.
Dame Beverley said the public needed assurances that both the letter and the spirit of the law were being followed by the public sector.
She expected the investigation to act as a "valuable health check" for the OIA which would delivery "very substantial benefits" for both requesters and holders of official information.
Guidelines for public submissions would be released early next year.