Some 1,400 Northern Ireland Water customers are to have a total of £53,000 reimbursed to them after mass billing mistakes were discovered.
But 360 other bill payers are to have some £130,000 clawed back after it emerged they weren't being charged enough over a six-year period.
NI Water has apologised after it emerged a total of 1,760 bills – some too much and some too little – had been wrongly charged over a lengthy period of time.
The Belfast Telegraph revelation comes just a month after it emerged that NI Water had to refund Altnagelvin Hospital in Londonderry more than £285,000 due to billing miscalculations.
The hospital had been supported in its claim by the Northern Ireland Consumer Council, and it's thought 19 other hospitals in the province are eligible for similar refunds.
The mistakes fall into two categories of billing by NI Water. The first involves 1,400 customers who were due to receive what is known as a 'Non Return to Sewer Allowance (NRTSA)' on their account which is 5% of the total bill.
The 5% allowance reflects water volume recorded through a meter which, because it is used in activities like cooking and drinking, is not discharged back into a sewer.
It was this water and sewage bill being miscalculated which resulted in the refund to Altnagelvin Hospital.
The internal billing system used by NI Water can only work backwards to a maximum of six years, so the bills – whether too high or too low – can't be calculated back any further. The number of billed outlets or the amount of money involved beyond that period is not known. The cost of administrating the error has also to be calculated.
The mistakes mean that the 1,400 who paid too much will share a £53,000 payback. But 360 outlets have been identified who did not have their water standing charge correctly applied to their account.
In letters sent by NI Water to customers – which have been seen by this newpaper – the charges were not applied at all.
As a result, the 360 affected customers are receiving bills for services dating back six years, in what totals £130,000.
An industry insider told the Belfast Telegraph that the numbers involved are expected to rise substantially as investigations into the billing of accounts continue.
A spokesman for NI Water said: "NI water would apologise for any inconvenience caused to our customers as a result of the necessary recalculation of these bills and is more than happy to discuss any issues which affected customers might have in relation to their accounts."
STORY SO FAR
Last month it emerged that the Altnagelvin Hospital in Derry was to receive a refund of more than £285,000 from Northern Ireland Water. Legislation that came into effect in 2008 changed how hospitals were charged for 'trade effluent'. However, the Consumer Council found that NI Water had only applied the new charges from 2010, and the hospital was entitled to a rebate.