(a) that a dealer has failed to furnish a return under section 32, in respect of any period before the assessment under section 45, or section 46, or section 48 for such period; and

(b) that on completion of such assessment, the said dealer has not paid full amount of net tax payable for such period by the prescribed date for furnishing the return, and is liable to pay interest under sub-section(2) of section 33, in respect of such period, such authority shall determine under sub-section (1) of section 50 the interest payable at the time of assessment under section 45, or section 46, or section 48 as the case may be, for such period.