Housing Minister Phil Twyford says an error from his office meant a meeting with another minister and the Auckland mayor was not included in his publicly released diaries.
Ministers for the first time this year publicly released their diaries as part of the government's commitment to open and transparent government.
A meeting between Mr Twyford, Environment Minister David Parker and Auckland mayor Phil Goff about Auckland's urban boundary appeared in Mr Parker's diary, but not in Mr Twyford's.
Mr Twyford's office then declined an Official Information Act request from National about the meeting on the grounds the information did not exist, which he said he was unaware of.
It has prompted questions to Mr Twyford in Parliament.
He denied any deliberate attempt to keep the meeting secret.
"The meeting that I had ... was to discuss the future of Auckland's rural urban boundary. Our policy is to get rid of that boundary, that's not a secret, we campaigned on it," he said.
He later told reporters he had only found out the entry was missing from his diary this morning, from a source he would not disclose.
"I was informed that there was a discrepancy between the published diary, which did not record the meeting I had with the mayor of Auckland, and the diary in my office.
"There was an error in transferring over the hundreds of entries in my diary to the published record of my diary."
It was corrected as soon as he became aware of the error, he said, and a check would be done to make sure there were no others.
An email released to National by Mr Parker's office said Mr Twyford had asked for the meeting to be seen as 'political' - which could exclude it from the public diaries - with no officials present.
National has criticised his handling of the disclosure, saying it doesn't fit with the pledge to be open and transparent.
Prime Minister Jacinda Ardern said there was "simply a clerical issue" with the diary release, which was then rectified.
"Ministers do proactively put out their diary; sometimes in doing that mistakes will be made."
She expected Ministers' offices to be as thorough as they could, but said they had hundreds of appointments and sometimes there would be the "odd mistake".
This did not merit asking for her minister's resignation, or demoting him, Ms Ardern said.
"I don't think people would expect that level of reprimand for something like that."