A dripping tap owned by Somerset County Council has run up a £40,000 bill.
The faulty tap was installed for a travellers' site and, in three months, leaked enough water to fill nine Olympic-sized swimming pools.
The former travellers' site between Middlezoy and Westonzoyland is no longer in use.
Somerset County Council has since negotiated the Wessex Water bill down to £20,000 and installed new meters which will be checked fortnightly.
Wessex Water said the case showed why water meters need to be regularly checked.