Within 15 days after the receipt of any statement of amount assessed under this act, the public utility may file with the commission objections setting forth in detail the grounds upon which the assessment is claimed to be excessive, erroneous, unlawful or invalid. The commission, after notice to the utility shall hold a hearing on the objections. If, after hearing, the commission finds the assessment is not excessive, erroneous, unlawful or invalid in whole or in part, it shall record its findings and transmit them to the public utility and again mail or serve a copy of the assessment upon the utility. Statements of assessment to which objections have not been filed, and statements of assessment and amended statements of assessment mailed or served after a hearing upon objections shall be paid not later than 30 days after their receipt. Assessments not paid when due shall bear interest at the rate of 1% per month. Statements of unpaid assessments together with interest thereon shall be recovered by the attorney general by appropriate action.