Phone records covering the month in which Act New Zealand leader John Banks visited internet tycoon Kim Dotcom are being withheld for police inspection.
The records, held by the Auckland Council, would be a key factor in the investigation into donations made to Mr Banks' campaign to be mayor in 2010.
Police are investigating complaints Mr Banks knew details of three anonymous donations, totalling $65,000, made to his campaign.
The New Zealand Herald asked the council for the dates, times and number of calls made to Mr Dotcom's mobile phone during June 2010. The council refused to release the bills, because it said it could interfere in a police investigation. A council spokesman said police had not yet asked for the phone bill.
