Altnagelvin hospital in Londonderry has been refunded more than a quarter of a million pounds in overcharged water bills.
Another 19 hospitals across Northern Ireland could be in line for a windfall worth millions after the Consumer Council secured its biggest ever repayment.
The 18-month investigation followed a change in how the north west’s main medical centre was charged for sewage disposal.
Council interim chief executive Aodhan O’Donnell said: “Not only is this the biggest refund the Consumer Council has ever secured, it is great to think this money could make a real difference for patients.
“And it is only the beginning of the story, because the work we have done with Altnagelvin’s representatives... has implications for all hospitals across Northern Ireland.”
A cheque for £285,616 was presented to Altnagelvin by Northern Ireland Water following the billing dispute.
It resulted from a law change in what is classified as standard sewage and what is trade effluent, like that produced by laundries or hydrotherapy pools.
This usually meant lower charges.
Northern Ireland Water revised its hospital charging policy and this was reflected in bills from 2010.
The Consumer Council argued that the changes should have been applied from 2008, when sewerage charges were introduced for businesses and public organisations, and that two-year delay plus technical changes contributed to the £285,000 refund.
Other hospitals have until the end of this month to make their own refund claims.
A Northern Ireland Water spokeswoman said a detailed review was conducted of the legislation and the resolution was satisfactory for all parties.
“The engagement process is coming to an end on 28 February 2014, however, NI Water remains committed to working with the other hospitals to ensure that any adjustments to their accounts are effected without delay on provision of the required evidence,” she added.
“NI Water welcomed the input of Consumer Council Northern Ireland during the investigations required to bring a positive outcome for our customer.
“However, a number of teams have been involved in this process and NI Water would like to commend the work of its own team in collating the information and ensuring it was checked and validated.”