Far more of Telecom's email accounts may have been compromised by a cyber attack than initially thought.
Telecom says over 60,000 customers using Yahoo! Xtra accounts will have to change their passwords after being hit by a spam attack last week.
The company on Saturday night cancelled the passwords of more than 15% of its 450,000 Yahoo! Xtra customers.
These customers joined 15,000 others who Telecom had already been contacting after infected messages were sent by hackers from their accounts.
Telecom said the company was told on Saturday night by Yahoo! about the further 60,000 accounts at risk.
Retail chief executive Chris Quin said Telecom moved quickly to ensure those accounts could not be accessed by anyone but the account holder.