Hong Kong Ordinances

DISTRICT COUNCILS ORDINANCE - SECT 10

An elected member or appointed member taken to have resigned on becoming an ex officio member

If a person holding office as an elected member or appointed member of a
District Council ("the first office") becomes entitled to hold office as
an ex officio member of the same or another District Council ("the second
office"), that person is taken to have resigned from the first office with
effect immediately before the date on which the person commences to hold
the second office.