In terms of Section 62(1) of the CGST Act, 2017, the proper officer is empowered to assess the tax liability on such registered person to the best of his judgment taking into account all the relevant materials which is available or which is gathered and issue an assessment order in FORM GST ASMT-13 within a period of five years from the date specified under Section 44 for furnishing of the annual return for the financial year to which the tax not paid relates.
(ICAI FAQ PUBLICATIONS 06-90-2107 ASSESSMENT OF NON-FILERS OF RETURNS: FAQ NO. 17)