Each port commission shall have power to raise revenue by a tax to be levied on all taxable property within such port district, not exceeding 2 mills in any one year on each dollar of the assessed valuation of the taxable property in such port district. The tax shall be for such number of years as approved by the electors of the cities or townships or of the county or counties and shall be levied and collected in the same manner now provided for the levy of state and county taxes under the general tax law, and shall be paid to the county treasurer having custody of the port district fund, to the credit of such fund, and such tax shall not exceed $1,500,000.00 in any one year. If the port commission is authorized under any present or future law of the state to establish special assessment zones and to raise revenue through the medium of special assessments for benefits within such zones, taxes so assessed shall be in excess of such 2 mill limitation. This act shall not authorize a county allocation board to allocate millage within the 15 mill limitation for capital construction purposes, except to meet any deficiency in the payments of principal or interest upon bonds regularly issued with the approval of the electors. Funds may be appropriated from regular millage for operating purposes only in an amount to be established by the board of supervisors, and the board of supervisors may also appropriate for any purposes moneys obtained as revenues from the operation of the port.