Irish Water say up to 30,000 customers still need to update their details with the utility to receive a refund of payments.
It says 25,000 customers have so far made contact to ensure they receive a refund cheque.
The deadline for changing details is Friday November 17th.
The refunds section on the website has had almost 160,000 hits over the past 10 days.
While almost 35,000 customers logged into the Online Account Management service to check their contact details are correct and to see how much they paid in domestic water charges.
And the utility says almost 280,000 customers have an online account - while those who do not can still sign up.
It says in the past 10 days, almost 7,000 new online accounts have been created.
Eamon Gallen is head of customer operations for Irish Water: "It is essential that we have the most up to date address if the refund cheque is to be sent to the correct location ensuring that customers get their refund as quickly as possible."
"Irish Water will issue almost one million refund cheques. We want to ensure that the cheques go to the right people at the right address.
"Based on CSO data we can estimate how many people may have moved since domestic billing was suspended.
"We know that those who sold a house will have informed us as they were required to under law to confirm that there were no bills outstanding on their property, but exactly number of those renting who paid domestic water charges and have since moved is less clear."
"It is inevitable given the volume and the fact that we have not issued bills in almost 18 months that our contact details for some customers will need to be updated.
"If people do not contact us we will send the cheque to the last address on file for the account holder."
Customers can contact Irish Water on 1850-448-448.