There were no significant changes in radiation levels outside the plant, he added.
Since a quake-generated tsunami struck Fukushima in March 2011, knocking out reactor cooling systems and sparking meltdowns, there have been four similar leaks from tanks of the same design.
But the latest leak was the worst from a tank in terms of volume, the TEPCO spokesman said.
TEPCO admitted the toxic water might contaminate groundwater and flow into the Pacific Ocean "in the longer term", but said it was working to avoid such a situation.
"We are transferring the contaminated water from a tank with a leakage problem to unbroken tanks, and retrieving leaked water and soil around it," the spokesman said.
"We are also beefing up existing earth-fill dams around tanks," he said, as the region braces for heavy rain later on Tuesday.
So far four tonnes of the spilled water had been retrieved since Monday evening when TEPCO started the recovery operation, the company said.
TEPCO has faced a growing catalog of incidents at the plant including several leaks of radioactive water, following the worst nuclear disaster in a generation.