A data tape containing information from 12,799 electricity bills has been lost. Billing information as well as names and addresses are included on the record of payment demands produced on August 10 last year, NIE Energy said.
It did not contain any banking details but the electricity company has written to customers.
The latest loss follows a string of similar lapses involving child benefit claimants and car drivers, among others.
NIE Energy managing director Stephen McCully said: "We fully appreciate that our customers will be concerned that any personal information regarding them is missing.
"However, while we have not been able to locate this tape, there is absolutely no evidence to suggest that it has fallen into the wrong hands."
The information commissioner's office has been alerted following the loss by contractors for NIE Energy who are in charge of bill processing.
The tape did not contain any bank, debit card or any other similar information that would be classified as sensitive under the Data Protection Act.
It held details such as account name and address, customer number, bill amount, previous payment and account balance from electricity bills dated August 10, 2009.
Northgate Information Solutions is contracted by NIE Energy to provide data management services including organising the backing up of billing information through its subcontractors, Liberata and Formscan.
Each day after electricity bills are produced Liberata transfers billing information by tape to Formscan to be backed up onto microfiche. It was during this process relating to a single day's electricity bills in August 2009 that the tape went missing.