Telecom New Zealand has started cancelling the passwords of about 60,000 Yahoo Xtra email accounts that it believes have been compromised following an online attack.
The users will need to enter new password information when they log in to their email account.
The 60,000 customers affected by Saturday night's password cancellation are additional to the 15,000 customers that Telecom has been contacting following last weekend's attack.
The move is aimed at protecting email customers and preventing information in emails being accessed. There is no evidence that this has occurred.
"We're taking this matter very seriously, and urge those whose passwords have been cancelled to create new passwords," Telecom retail chief executive Chris Quin said.
"However, it's advisable for all others that have not changed their password to do so immediately." This should be done on computers and mobile devices.
"We continue to be sorry for any distress caused or inconvenience this has caused, and reinforce that in today's online world, regular password changes are an important need."
Telecom has not revealed how the previous accounts were breached, but Yahoo's email system was targeted earlier this year in a related attack. At the time, attackers used a vulnerability in its developer blog that had not been patched for eight months.
Although this vulnerability has been closed, many Yahoo members still appear to be sending the "work from home" emails that were part of the original attack on Yahoo, without their knowledge. This indicates that scammers still have access to a number of accounts from the attack using the developer blog, or that they may have found a second vulnerability.
Michael Lee contributed to this report.