Penal interest rates apply to overdue public deposits, with a higher compoundable rate for small depositors on unpaid maturity. A penal rate of interest is prescribed for public deposits matured and claimed but remaining unpaid; ordinary public deposits attract an elevated penal interest for the overdue period, while deposits by a small depositor attract a higher penal rate that is compoundable on an annual basis, with 'small depositor' defined by reference to the Act.