Penalty relief for voluntary and timely full disclosure enables administrative reduction or waiver where cooperation and payment arrangements are present. The Principal Commissioner or Commissioner may reduce or waive penalties under section 439 where the taxpayer voluntarily and in good faith fully discloses income before detection, cooperates with enquiries, and pays or arranges tax or interest; deemed full disclosure applies where assessed variance does not attract penalties; prior approval from senior authorities is required where disclosed amounts exceed a prescribed threshold.