The authority may levy in any year, in addition to the tax required for the payment of bonded indebtedness, a tax for the purpose of providing funds for administration expenses of the authority and such other purposes of the authority as may be determined to be necessary by the board of commissioners. The tax shall be uniform and shall in no event exceed 1/2 mill on the combined state equalized valuation for all governmental units comprising the authority, and shall be levied only after the adoption of a resolution therefor by unanimous vote of all the members of the board of commissioners of the authority. The tax shall be levied and collected in the same manner as taxes for the payment of bonded indebtedness are levied and collected under the provisions of section 16 of this act.