In this Chapter, references to a storm overflow of a sewerage undertaker are to any structure or apparatus—
which is comprised in the sewerage system of the sewerage undertaker, and
which, when the capacity of other parts of the system downstream or of storage tanks at sewage disposal works is exceeded, relieves them by discharging their excess contents into inland waters, underground strata or the sea.
References in this Chapter to discharges from a storm overflow do not include discharges occurring as a result of—
electrical power failure at sewage disposal works,
mechanical breakdown at sewage disposal works,
rising main failure, or
blockage of any part of the sewerage system downstream of the storm overflow.
Section 17BA(7) (meaning of sewerage system of a sewerage undertaker) applies for the purposes of subsection (1).