(3) Where any employer is in default in paying the compensation due under this Act within one month from the date it fell due, the Commissioner may direct that, in addition to the amount of the arrears, simple interest at the rate of six per cent. per annum on the amount due together with, if in the opinion of the Commissioner there is no justification for the delay, a further sum not exceeding fifty per cent. of such amount, shall be recovered from the employer by way of penalty.]