If any Councillor is by reason of his death, resignation or removal or otherwise, is unable to complete his full term of office, the vacancy so caused shall be filled up by election or appointment, as the case may be, of a person under the provisions of this Act and the person so elected or appointed shall hold office for the unexpired term of his predecessor:
     Provided that no election for filling up a casual vacancy shall be held if the vacancy occurs within a period of six months preceding the date on which the term of office of the Councillor expires.