(1) As soon as practical after a return is filed, the department shall examine it to determine the correct amount of tax. If the department finds that the amount of tax shown on the return is less than the correct amount, it shall notify the taxpayer of the amount of the deficiency determined.
(2) If any person fails to file a return or has improperly purchased motor fuel without the payment of tax, the department may estimate the person's liability from any available information and notify the person of the amount of the deficiency determined.
(3) The amount of the deficiency determined shall constitute a final assessment together with interest and penalties thirty days after the date on which notice was mailed to the taxpayer at his or her last-known address unless a written protest is filed with the department within such thirty-day period.
(4) The final assessment provisions of this section shall constitute a final decision of the agency for purposes of the Administrative Procedure Act.
(5) An assessment made by the department shall be presumed to be correct. In any case when the validity of the assessment is questioned, the burden shall be on the person who challenges the assessment to establish by a preponderance of the evidence that the assessment is erroneous or excessive.
(6)(a) Except in the case of a fraudulent return or of neglect or refusal to make a return, the notice of a proposed deficiency determination shall be mailed within three years after the last day of the month following the end of the period for which the amount proposed is to be determined or within three years after the return is filed, whichever period expires later.
(b) The taxpayer and the department may agree, prior to the expiration of the period in subdivision (a) of this subsection, to extend the period during which the notice of a deficiency determination can be mailed. The extension of the period for the mailing of a deficiency determination shall also extend the period during which a refund can be claimed.