Wis. Stat. § 114.76: Bonds exempt from taxation.

The state covenants with the purchasers and all subsequent holders and transferees of bonds issued by the authority, in consideration of the acceptance of any payment for the bonds, that its fees, charges, gifts, grants, revenues, receipts, and other moneys received or to be received, pledged to pay or secure the payment of such bonds shall at all times be free and exempt from all state, city, county, or other taxation provided by the laws of the state.