Irish Water customers whose address or other details have changed since they paid water charges are required to update their information with the company by 5.30pm in order to ensure they receive a refund.
Almost one million households who paid the charges are due to receive a cheque to their registered address from Irish Water refunding the total amount paid.
It is estimated that €173m will be refunded to customers, with the company having said that 30,000 cheques would be issued each day.
It had been announced that all customers would receive their refund before the end of the year, if legislation was enacted by the middle of October.
However, the legislation required to approve the refunds was only approved by the Dáil earlier this month.
Customers who have moved since they paid the charges are asked to change their details either via a dedicated webpage or by contacting a CallSave number - 1850 448 448.