Wis. Stat. § 234.28

The state covenants with the purchasers and all subsequent holders and transferees of notes and bonds issued by the authority, in consideration of the acceptance of any payment for the notes and 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 notes or bonds shall at all times be free and exempt from all state, city, county or other taxation provided by the laws of the state.