Self-assessment requirement: pay tax due promptly or face penalties, subject to pre-levy hearing rights and deemed assessment credit. Substitution establishes a self-assessment regime requiring an assessee who files a return under section 139 to pay tax shown as payable, after crediting tax already paid, within thirty days of filing where the payable amount exceeds the statutory threshold; such payment is to be treated as payment toward any subsequent regular assessment, and failure to pay within that period may attract penalties directed by the Income-tax Officer, including successive penalties for continuing default, subject to an overall cap, with a reasonable opportunity of being heard before levy.