Irish Water says 42,000 people have yet to receive a refund from the utility.
It says this is because these customers have yet to confirm their details in order to receive their cheque.
This amounts to €5.17m in outstanding monies to be refunded to 4% of customers.
In a statement, it says: "Currently there is no end date by which customers have to claim their refund.
"The scheme remains open and there are details on the Irish Water website".
"There may be a small percentage of customers who have moved abroad or passed away and who will never seek a refund but the funding allocation is there and will remain available to them or their estate."
It adds that these funds are kept in a separate account.
Any customer who believes that they are entitled to a refund and has yet to receive it is advised to contact Irish Water on 1850-448-448 for advice.
Last week, the utility was given the go-ahead to charge households that use too much water.
Some 80,000 households currently use in excess of the annual allowance of 213,000 litres.
Many of these have leaks and actually account for almost 40% of all domestic water consumption in Ireland.
The Commission for Regulation of Utilities (CRU) gave Irish Water the green light to bill people if they are using too much water.
If a household is using more than they should be, they will get a warning letter before Christmas.
They will then have 12 months to address any possible leaks or reduce their usage before they are billed.