The tax imposed by sections 148.120 to 148.230 shall be due and payable upon the last day upon which a return must be filed under section 148.160. Upon the filing of such return the full amount of any tax as computed by the taxpayer shall be paid to the director, who as soon as is practicable thereafter shall examine it and determine the correct amount of the tax. If the director determines that the taxpayer has paid a tax in excess of the amount lawfully due, the director shall permit a credit. If any tax due hereunder is not paid when due, the taxpayer shall be required to pay as part of such tax interest thereon at the rate determined by section 32.065 from such time.