Complex sentence: Five further reservoirs were subsequently built underground, the latest of which was completed in 1977, and are currently used to maintain supply to the Onehunga area and to the reservoirs atop Maungawhau/Mount Eden.
Split into simpler sentences: Five further reservoirs were subsequently built underground, the latest of which was completed in 1977. They are currently used to maintain supply to the Onehunga area and to the reservoirs atop Maungawhau/Mount Eden.