Interest on penalty accrues monthly; regulator may reduce or waive it and adjust on appellate reduction. Failure to pay a penalty within the demand notice period triggers simple interest at one per cent on the outstanding amount for each month or part month from the day after expiry until payment; the Commission may reduce or waive interest for defaults beyond the payer's control, and if a court reduces the penalty the interest is reduced accordingly and excess interest refunded under regulation 14.