While that may not much (45 tonnes), the water may contain extremely high levels of beta-nuclides like strontium; according to TEPCO, the density of strontium could be as high as 100,000 becquerels per cubic centimeter. That's 100 million becquerels per liter.
The leak was found at the post-Kurion/SARRY treatment facility that condenses the treated water (TEPCO calls it "evaporative condensation apparatus).
Here's TEPCO's handout for the press on December 4, 2011.
Here's from Yomiuri Shinbun (12/4/2011; since I'm at the terminal in the public library I cannot quote the original Japanese as it won't allow copying. But Yomiuri Shinbun tends to retain the link for a long time):
TEPCO announced on December 4 that about 45 tonnes of contaminated water leaked at the water treatment facility at Fukushima I Nuclear Power Plant and part of the water has leaked outside the facility.
The water may have gone into the drains and leaked into the ocean, according to TEPCO, but "It would be a small amount even if the water had leaked, and very little effect [on marine life]", says the company.
The leak was found at the evaporative condensation facility, which the water goes after being treated to remove radioactive cesium [Kurion and SARRY]. The leaked water is 5 centimeter deep inside the facility. On the surface of the water, the radiation level measuring gamma rays is 1.8 millisievert/hr, but it is 110 millisieverts/hr measuring beta rays. In order to mop up the water in the facility, the workers would have to do it without coming in contact with the water to avoid exposure to beta-ray radiation.
According to TEPCO, 100,000 becquerels/cubic centimeter of strontium, a beta nuclide, may be in the water. The level is 100,000 to 1 million times the safety limit for seawater.