Its annual report said while companies continued to deliver safe, reliable supplies, six out of 21 companies did not meet their leakage targets.
These were Southern, Northumbrian, Veolia Central, Dee Valley, Cambridge and Yorkshire which have been told to implement fresh measures to tackle the problem.
This is the first time so many water companies have failed to hit their annual leakage targets.
Ofwat said it intends to get tough on leakage. The report said: “We have focused on what these six companies need to do to restore their performance and we have increased their reporting requirements while they do so.
“In the case of Southern, Northumbrian and Welsh Water, we are requiring them to deliver action plans to restore stable serviceability.
Separate concerns about Welsh Water stem from contracting out operation of its water assets, which have now been brought in-house.
The report added: “During the year, we carried out an investigation into Cymru’s understanding and management of its assets, and its management of contractors delivering services on its behalf.
“It is taking action to strengthen its systems and processes and make sure that it has a full understanding of its assets.”