Wis. Stat. § 905.02

A person, corporation, association, or other organization or entity, either public or private, making a return or report required by law to be made has a privilege to refuse to disclose and to prevent any other person from disclosing the return or report, if provided by law. A public officer or agency to whom a return or report is required by law to be made has a privilege to refuse to disclose the return or report if provided by law. No privilege exists under this section in actions involving false swearing, fraudulent writing, fraud in the return or report, or other failure to comply with the law in question.