There were several times that the colonial government had to resort to severe water rationing in the 1950s-1960s as the water supply from the reservoirs is insufficient due to little rain and the demand of water grew rapidly due to rapid increase in population. Then the HK government decided to purchase water from the PRC to secure supply. Today 70% of HK's water come from PRC.
The large reservoirs you mentioned were actually built after the HK government started buying water from PRC. However, there is no way that the HK island can be self- sufficient for water as there is no suitable space that can be developed into reservoirs and the rainfall is not suficient to support the population.
By the way, are you still in HK now? Perhaps we can have a mini- AH.com gathering?