Telecom is refusing to say what it has found out about unauthorised access of its customer database, known as Wireline, other than to say its investigation is progressing well.
The company believes a sales firm working for its competitor, Slingshot, has had unauthorised access to names, addresses and other data.
Credit card and other financial information wasn't accessible.
Telecom Retail's chief executive, Alan Gourdie, says the matter is being taken extremely seriously, and a security review of Wireline has begun.
The Privacy Commissioner's office is also continuing its investigation, and has declined to comment.
