Mr McMahon added the Testwood site was “very old” and £250m was being invested into “bringing it up to standard” over the course of seven years.
Explaining what had happened to supplies, he said: “Through some of those planned works on Tuesday, when we were starting up the site, we were having problems getting it going and we had some dirty water in that part of the site.
“We couldn’t let that go forward, we can’t put it into supply, it’s illegal, we wouldn’t do it and the site shut down, and then we had to get rid of that water and that took about 24 hours.
“Also, because it’s an old site it takes time to get it working and that’s what we’ve been working very hard on.”