The local residents went to clean up the water tank in Neliant area of Gwadar district, Deputy Commissioner Gwadar Abdul Hameed Abro said.
During the cleaning process some people fell unconscious after inhaling poisonous fumes. Three of them, including a child, later died at a hospital, Abro said.
Last month, five persons had died after inhaling poisonous fumes in a water tank in Nodzai in Turbat district.
And before that six children and three women died in their sleep after inhaling carbon monoxide emitted from an electricity generator installed in their room in Pishin district.