The first lake tapping had been done in 1999. The dam was commissioned in 1962.
Maharashtra Chief Minister Prithviraj Chavan, Deputy Chief Minister Ajit Pawar, and Water Resources Minister Sunil Tatkare were present on this occasion.
The tapping is aimed at generating additional 1,185 million units of electricity a year.
"Now, the additional water stored in Koyna dam can be utilised as the second lake tapping was conducted at a depth which was 12 metres lower than the first lake tapping," Chief Engineer (Civil-Hydro Projects), Deepak Modak said.
"The first lake tapping was done with the help of Norwegian technology, while this one was carried out by experts of the state Irrigation Department," he said.
The Koyna project generates 1960 MW of power. The tapping will also provide additional 15 TMC water to the Krishna Valley projects.