Auckland Mayor Phil Goff says he has no idea who is the source of a leak from inside his own council.
It follows growing discontent among some councillors about Goff's handling of a pre-feasibility report into a downtown stadium.
A letter this week to Goff from nine councillors, leaked on Tuesday, cited concerns around bullying and the refusal to release the $923,000 report to elected members.
The Chief Ombudsman announced on Thursday he had launched a formal investigation following complaints from two councillors about Goff's handling of the report.
"In our view this behaviour falls far short of the sort of transparency and inclusiveness we would expect in such a significant matter as this is concerning a potential $1.5 billion stadium," the councillors said in the letter.
Goff told Newshub this morning he did not know who had leaked the letter.
"If I had evidence of the person who leaked then I would be confronting that person right now on it and rightly so."
"It's not a case of punishing them ... I am not resentful of it being played out in the media but there are other ways," Goff said.
In the letter councillors wrote the report emailed to them was "so heavily redacted so as to make the value of these documents in such a heavily censored form, questionable to say the least".
The mayor said that he would not be sending out electronic copies of the report because he was worried sensitive information would be leaked.
"Too often in the past when I've sent something out that's confidential it's been in the public arena within minutes," he said.
Last week Auckland Councillor John Watson revealed a standing joke among a number of councillors.
"If you want to find out what's happening with Auckland Council, wait to see the front page of the Herald the following day," Watson said.