(1) When assessments of benefits have been made in drainage districts organized either under general or special acts, the property owner shall have the right to pay the assessment in full within sixty (60) days after the formation of the district, but if he does not avail himself of this privilege, the assessment of benefits shall bear interest at the rate of six percent (6%) per annum and shall be payable only in installments as levied.
(2) The interest need not be computed until necessary to be sure that the collections have not exceeded the total amount of benefits and interest, or the interest may be first collected.
(3) However, this section shall not apply to districts heretofore organized in which interest on bonds or other borrowed money was calculated as a part of the cost of construction and included in the assessment of benefits.