Banks get daily requests from the police for personal banking information, and one says it is influenced by law enforcement interest when it assesses customers.
The banks supply the police with information on the basis they are told that doing so assists with "maintenance of the law" - an exception in the Privacy Act to confidentiality rules.
The Weekend Herald has found financial information which banks promise to keep confidential is passed to police and then to other agencies.
In some cases, it ends up overseas for use in foreign court orders.
Account details, contact information and even financial information can be handed over to police without a search warrant.
The arrangement has been revealed as a result of inquiries into the Kim Dotcom case in which Kiwibank appeared to use a police request as a sign of impending trouble for the tycoon, rejecting a loan application.
The Weekend Herald has identified other police operations involving the same branch of police - the Organised and Financial Crime Agency of New Zealand - under which financial information about targets of interest is passed to the Ministry of Social Development and Inland Revenue.
In one case, details about a group under investigation went to Social Development which then reviewed benefits those people were receiving. In another case, financial details were passed to Inland Revenue, which then carried out a tax assessment.
Kiwibank spokesman Bruce Thompson said police requests for information influenced the bank's view of customers.
"Kiwibank takes any approach from police seriously, and as a prudent lender, would consider any such approach as part of its overall assessment of any banking relationship.
"It does not influence the bank's position one way or another but is taken into account as part of the bank's holistic assessment of a customer's character and suitability for lending or general banking services."
He said police sought information from banks on a daily basis.
A spokeswoman for the Privacy Commissioner said banks could refuse to supply the information.
"One key thing to note is that the Privacy Act provides a discretion to those agencies."
Banking Ombudsman Deborah Battell said she had not investigated any complaints about the process.
The practice is used by the police financial intelligence unit to obtain personal details without any legal compulsion or court order. It allows officers to harvest financial information by placing the decision to release information entirely on the banks.
The Banking Association's code of practice states: "We have a strict duty to protect the confidentiality of all our customers' and former customers' affairs. We are also obliged in our dealings with our personal customers to observe and comply with the Privacy Act 1993."
Police would not say how often banks gave them information.
The head of the financial crime group, Detective Superintendent Peter Devoy, said the details were being withheld because it related to information "subject to an obligation of confidence".
It was in the public interest that such information should continue to be supplied.
A police headquarters spokeswoman said it was not an issue for the police to comment on because the decision to supply information was made by the banks.
"We lawfully obtain information for law enforcement purposes and on occasions we lawfully share information for law enforcement purposes."
Tactics employed by Ofcanz have come in for criticism in other investigations.
It is under scrutiny in the Kim Dotcom case. The search and seizure on the day of his arrest has been deemed illegal by the courts.
Cops' query hurt Dotcom loan
Kim Dotcom's application for a $4 million mortgage was declined after police asked Kiwibank for his records.
Mr Dotcom's lawyers were arranging a mortgage to buy a house next to his North Auckland mansion.
Kiwibank approved the loan then withdrew the offer just before settlement and days after police sought information on his finances.
Kiwibank spokesman Bruce Thompson said the loan was rejected after a "simple internet search" turned up troubling information. But he also said the query from the police could not be ruled out as a factor in the decision to decline the loan.
Detectives working at the FBI's behest harvested Mr Dotcom's financial information from Kiwibank, ANZ, ASB, HSBC and the BNZ, according to court records. There was no formal legal request for assistance until November 25, 2011.
In the three months before the request, according to information held at the Auckland High Court, police and the FBI met several times to discuss the internet tycoon.
Notes taken at meetings between police, Crown Law, the FBI and the GCSB by Ofcanz deputy director Detective Inspector Grant Wormald show the earliest recorded planning meeting included the head of the financial intelligence unit, Detective Superintendent Peter Devoy. The meeting on August 29, 2011 included discussion about seizing assets.
Efforts to collect the information took place through October and November 2011. The analyst who led the work, Fiona Milne, briefed US Department of Justice lead cybercrime prosecutor Jay Prabhu and FBI lead agent Michael Poston at a meeting on October 31, 2011. Notes of the meeting show she helped with a "presentation in regards to NZ financial institutions related to subjects of the investigation".
The meeting also discussed the "procedure/plan for searching financial records in NZ".
Police, FBI and Crown lawyers met again on November 4 and discussed the FBI getting an "international restraining/seizure warrant".
Police got leads on people and accounts through emails between Megaupload staff obtained on US search warrants and passed on by the FBI. Information including addresses, account numbers and account balances was handed over.