The Ombudsman's office found Associate Education Minister John Banks wrongly withheld information about how proposed charter schools would be funded.
The Post Primary Teachers Association made a request under the Official Information Act (OIA) about resourcing, capital and operating funding for charter schools. The information was withheld in February last year.
Ombudsman David McGee has recommended that Mr Banks release the information.
He advised that there was no good reason to withhold information on the funding of charter schools.
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"The proposal to create a new type of school within the New Zealand educational system is a significant step, and is - in my opinion - a circumstance in which the disclosure of policy advice at this stage in the process is required by section 5 of the OIA when considering the accountability and participation purposes of this enactment.
"Ministers cannot be held fully accountable for the proposals they are putting forward, unless the relevant information is in the hands of the public."
Mr McGee also found that the public could not participate in the "making and administration of laws and policies" if they are not informed of key elements of the Government's proposals.
A spokeswoman for Mr Banks said when the request was made, the charter schools working group had not been set up, and no funding decisions had been made.
The education and science committee heard a high number of submissions on proposals under the Education Amendment Bill to set up charter schools.
Charter schools would get money for set up costs and property funding that their private-sector backers would be able to keep if a school folded.