The workers miscalculated the tank's capacity as it was tilted on unlevel ground, plant operator Tepco said.
It said around 430 litres (100 gallons) of water may have leaked from the tank, and could have flowed into the sea.
The plant has experienced several leaks since being crippled by an earthquake and tsunami in 2011.
One of the largest leaks took place in August, when Tepco discovered a leak of at least 300 tonnes of highly radioactive water at a different part of the plant.
The latest leak was discovered by workers late on Wednesday.
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| Image: bbc |
Tepco official Masayuki Ono said: "We would like to apologise that we have to announce that we've had another leak in our tanks today."
"This is partly because we've had to fill our tanks to the brim in order to deal with the difficult management of rain water overflow following [a typhoon]," he added.
Chief Cabinet Secretary Yoshihide Suga said that Tepco had failed to deal with the leaks successfully.
"It's actually leaking so of course we can't say that [Tepco] have been properly dealing with the issue. It should not be leaking at all," he said.
The 2011 disaster knocked out cooling systems to the nuclear plant's reactors, three of which melted down.
Water is now being pumped in to cool the reactors, but storing the resultant large quantities of radioactive water has proved a challenge for Tepco.
In September, the government said it would invest hundreds of millions of dollars into building a frozen wall around the plant to stop the leaks.
Source: http://www.bbc.co.uk/news/world-asia-24377520
