Wis. Stat. § 605.09: Restrictions on private insurance.

After a lawful vote of the local governmental unit to insure under this chapter, no such unit may pay out any money to any private insurer nor incur any indebtedness against the unit to a private insurer for any insurance on any property of the unit or for which the unit may be legally liable if such insurance is available under this chapter, unless it is approved by the commissioner as necessary or unless it is insurance on personal property which the unit by resolution filed with the commissioner has decided to insure in insurance companies authorized to do business in this state.