Any employer against whom an assessment is made may, within 30 days after date thereof, file with the Commissioner a petition for a hearing before a referee appointed for such purpose, which petition shall set forth specifically and in detail the grounds upon which it is claimed the assessment is erroneous. Hearing or hearings on the assessment shall be held by the referee at such times and places as may be provided by rules and regulations of the Board and due notice of the time and place of such hearing or hearings shall be given by ordinary or certified mail to the petitioner. After hearing as above provided, the petitioner shall be promptly notified by ordinary or certified mail of the findings of fact, conclusions, and decision of the referee. The decision of the referee shall be final unless the employer or Commissioner makes application for review thereof by the Board within 30 days after date thereof or unless the Board, on its own motion within said period, initiates a review thereof. (Amended 1959, No. 329 (Adj. Sess.), § 22, eff. March 1, 1961; 1961, No. 210, § 8, eff. July 11, 1961; 1989, No. 8, § 2; 1991, No. 82, § 3.)