The Tansa lake, a key source of potable water for the metropolis, started overflowing on Thursday afternoon due to heavy rains in the last couple of days, the Brihanmumbai Municipal Corporation (BMC) said.
Tansa, located in adjoining Thane district, is the second water body after the Tulsi lake to get filled to the brim.
"The Tansa lake started overflowing today at 2.50 pm. Last year, this lake had overflowed on July 17 at 6.15 am," a senior civic body official said.
The Tulsi lake, another reservoir suppling water to Mumbai, had started overflowing at 7 pm on July 12.
The official said heavy downpour, mainly on Wednesday and partly on Thursday, helped in significantly increasing level in all the seven lakes suppling water to Mumbai.
The total water stock in all the seven reservoirs taken together stood at 57.86 per cent of their storage capacity as on July 25, he added.
With the rising level in these lakes, the BMC last week rolled back the 10 per cent water cut that was in force since November last year.