Hackers are using New Zealanders' email accounts to send out thousands of spam messages, and Telecom admits it still doesn't know the cause or scale of the problem.
Computer users with Yahoo! Xtra email accounts are being advised to immediately delete any suspicious emails they receive, even if they apparently come from known contacts.
On Friday, Telecom said it was working with Yahoo! to investigate the issue, and suspicious emails, often with "Hi" or "Hello" in the subject line, have flooded email inboxes since.
Yesterday a Telecom spokeswoman said that the company was urgently seeking answers to the issue.
"At this point we don't have confirmation of the cause or the scale of the problem.
"In the meantime our advice to customers is that they should mark suspicious looking emails as junk/spam and should never click on the links contained in them, even if the emails come from a known contact."
If a customer thinks their account may have been used to send out spam emails, then they should change their password as soon as possible.
Advice on how to do this can be found on Telecom's website, www. telecom.co.nz/emailpasswordhelp.
A total of 87,000 accounts were compromised in an attack in February last year.
At the time, Paul Brislen of Telecom Users Association New Zealand, said Yahoo! had an "utterly awful" security record and equally bad customer service.