Wis. Stat. § 76.18
The proceedings of the department shall be presumed to be regular and the determination of the department shall not be impaired, vitiated or set aside by any court upon any grounds not affecting the substantial justice of the tax. The provisions in this subchapter prescribing a date or period at or within which an act shall be performed or determination made by the department shall be deemed directory only, and no failure to perform any such act or make such determination at or within the time prescribed therefor shall affect the validity of such act or of any determination made by the department, unless it appears that substantial injustice has resulted therefrom. Nothing in this subchapter shall preclude the court in any proceeding before it under s. 76.08 from redetermining the assessment of the property of any company defined in s. 76.02 when in the judgment of the court the assessment should be substantially less or more than the assessment as determined by the department.
Wis. Stat. § 76.18