The Tokyo Electric Power Company (Tepco) had originally said the radiation emitted by the leaking water was around 100 millisieverts an hour.
However, the company said the equipment used to make that recording could only read measurements of up to 100 millisieverts.
The new recording, using a more sensitive device, showed a level of 1,800 millisieverts an hour.
The new reading will have direct implications for radiation doses received by workers who spent several days trying to stop the leak last week, the BBC's Rupert Wingfield-Hayes reports from Tokyo.
In addition, Tepco says it has discovered a leak on another pipe emitting radiation levels of 230 millisieverts an hour.
The plant has seen a series of water leaks and power failures.