Tepco has indicated there was a possibility that the contaminated water from the tank had diluted with rainwater and seeped into the soil, the Japan Times reports.
According to the report, workers at the site had detected radiation of 650 becquerels per liter from samples of a monitoring well dug near the damaged tank.
Tepco had earlier agreed that it discovered the leak, but was uncertain where most of the contaminated water was going.
The company has now estimated about 300 tons of highly radioactive groundwater is entering the Pacific Ocean daily, the report added.
Japan had earlier announced it will spend an estimated 473 million dollars to build an "ice wall" around the Fukushima nuclear plant to stop radioactive water leaks into the Pacific Ocean.
The plant was crippled by the March 11, 2011 earthquake-cum-tsunami that knocked out the reactors cooling systems, leading to the meltdown of at least three of them.
The Tokyo Electric Power Company (Tepco) had started storing contaminated water to cool the reactors, but the storage of large quantities has proved a challenge for the company, the BBC report added.
Reports of fresh leaks from the pipes and radiation hot spots on the ground, even where no water is evident, has raised concern of radioactive water seeping into the ground.