Modak Sagar, one of the seven reservoirs which supply water to Mumbai, started overflowing on Tuesday night following incessant rains in the catchment area, said a civic official.
Tulsi and Vihar lakes, which are located within the city limits, have already filled up.
Modak Sagar on the Vaitarna river in Thane district, which has a total storage capacity of 15.8 TMC, overflowed around 9.30 pm, said an official of the Brihanmumbai Municipal Corporation.
Last year the dam was full by July 26, whereas the year before it had overflowed on July 15, 2019.
Seven reservoirs -- Bhatsa, Upper Vaitarna, Middle Vaitarna, Tansa, Modak Sagar, Vihar and Tulsi -- supply water to Mumbai.
The BMC has implemented 20 per cent water cut in the city as the dams supplying water to Mumbai had less than 50 percent water stock as of July end.
Meanwhile, BMC said it will review the decision on 20% water cut in the city. "Water cut likely to be relaxed for Mumbai, in view of good rains in catchment areas in August," tweeted KS Hosalikar, deputy director-general of IMD, Western region.
With a good rainfall in August, the seven dams have 83 per cent water stock, officials said.