Auckland mayor Len Brown used his council phone to make nearly 1,400 personal calls and texts to his mistress Bevan Chuang.
He also failed to declare more than $39,000 in free hotel rooms and upgrades.
A report into the pair's affair, released this afternoon, said Mr Brown used his council phone to make 1375 calls and texts to Ms Chuang between November 19, 2010 and October 21, 2013.
Ms Chuang said all of the calls and texts were of a personal nature.
The mayor also received nine free hotel rooms which he didn't declare, valued at $6,130.
He also failed to declare 54 hotel upgrades valued at $32,888.50.
Mr Brown made one reimbursement of $263 for personal phone use on October 2012 but no subsequent reimbursements have been made.
Auckland Council chief executive Doug McKay said the council was unable to quantify the amount Mr Brown would need to pay to reimburse the calls.
"There were some instances where the relevant council policies were not fully met, or the use of resources raised questions. These include the amount the mayor reimbursed to council for costs incurred for personal mobile usage," Mr McKay said.
The review found mayor Mr Brown didn't show any improper favourable treatment towards Ms Chuang, including the reference he provided for her job at Auckland Art Gallery.
"I supported the Chief Executive's decision to commission this review. I welcome the conclusions, and I believe they draw a line under the questions that the review sought to address."
Mr Brown admitted he should not have accepted free rooms, and should have disclosed that he had in October.
"This was an error of judgement and I apologise to the people of Auckland," he said.
"I remain totally focused on the issues that matter most to Aucklanders, including improving our transport system, tackling Auckland's housing crisis and continuing to invest in our future."
Mr Brown said the report included details in relation to his and his family's private use of accommodation that were not within the scope of the investigation but that he needed to address.
"The report notes that over a three-year period I, and my family, stayed privately in hotels in central Auckland on a number of occasions. The arrangements for these hotels were made privately in most cases, and in all cases payment was made privately.
"My reason for staying in the central city is that I often work until late in the evening - attending meetings, functions or civic events - and I start work early the next morning, often for media interviews or breakfast events. A significant number of these rooms were also booked and used privately by me and my family.
"I was not charged for nine of these hotel rooms, including one occasion in relation to Ms Chuang."
Mr Brown said the report noted he never used council resources for private accommodation or in relation to Ms Chuang, and does not hold a council credit card.
"However, I accept that as Mayor I am subject to a higher standard of public accountability, and in this context I should not have accepted the free rooms offered to me, and should have disclosed this fact when I was asked about it in October."
Mr McKay revealed the Ernst & Young report at a media briefing earlier this afternoon.
The report had not been made available to the media before the briefing.
The review's terms of reference relate to any use of council resources in respect of Mr Brown's relationship with Ms Chuang that contravene council policy, and any improper preferential treatment given to Ms Chuang as a council employee, contractor or adviser.
It was also able to probe "any other issues that the reviewers or chief executive considers relate to, or arise out of, the above matters".
Mr Brown took legal advice on the report after being given a draft version on Friday.
Since receiving a draft copy of the report seven days ago, Mr Brown has received legal advice before providing feedback yesterday.
Mr McKay has received legal advice from Crown Solicitor Simon Moore, QC.
• The mayor used his council phone to make 1,375 calls and texts to Ms Chuang between 19 November 2010 and 21 October 2013.
• The calls and texts made up 10 per cent of his total phone use.
• Mr Brown advised two-thirds of those were of a "personal nature".
• He made one reimbursement of $263 on 25 October 2012.
• Ms Chuang attended several functions as the mayor's "translator". She is not on the council's list of official translators.
• The mayoral vehicle and driver were used to take Ms Chuang home after a number of functions.
• Mr Brown received nine free hotel rooms not registered as gifts, valued at $6,130.
• He received free upgrades to hotels not registered as gifts valued at $32,888.50.
• Other gifts not registered included an NRL Grand Final ticket and an iPad.
• Council paid for Mr Brown's dinner with a personal friend in Shanghai.
• The report's authors were "unable to identify" the security guard who interrupted the mayor and Ms Chuang having sex.