Hundreds of New Zealanders have had their email accounts hijacked by "savvy" spammers.
It isn't known how the Yahoo Xtra email security was breached. But once it was, emails were sent to everyone on the users' contact lists asking them to click on a link.
Telecom - which uses Yahoo for its email service - said it was "a suspected phishing issue".
Phishing is a tactic used by scammers pretending to be legitimate sources to get confidential information such as passwords, usernames or even credit card details.
Telecom spokeswoman Jo Jalfon said the problem arose on Saturday afternoon. "Some customers reported experiencing bounced emails and noticed emails were being sent to their contact list after opening suspicious emails."