Tokyo Electric Power Co.'s inexact monitoring of contaminated water stored at Fukushima No. 1 may have led the company to overstate last week's storage tank leak, according to the Nuclear Regulation Authority.

The loss of 300 metric tons that Tepco reported was based on an assumption that the tank had been full before the leak, Shunichi Tanaka, chairman of the NRA said Wednesday in Tokyo. That assumption may not have been reliable because there was no gauge measuring the tank's water level, he said.

"We have no idea whether it's actually 300 tons that leaked," Tanaka said. "We need to look into this issue more."