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Computer users with Yahoo! Xtra email accounts should delete any suspicious emails they receive - even from people they know - as Telecom investigates a possible virus.
A Telecom spokeswoman said the company was working urgently with Yahoo! to investigate the issue that hit some users today.
People have reported receiving numerous suspicious emails with 'Hi' or 'Hello' in the subject line and a link to click.
"If a customer thinks that their Yahoo! Xtra account may have been compromised - that is, if they think their account has been used to send spam - we advise them to change their password as soon as possible to re-secure the account."
Advice on how to do this could be found on Telecom's website, www.telecom.co.nz/emailpasswordhelp.
"It's important that they choose a strong password ... and that they don't reuse a password that they have used previously," the spokeswoman said.
The New Zealand Herald has received numerous emails from readers reporting suspicious emails coming into their inbox.
Last month some Yahoo! Xtra customers were unable to access or properly use their email for up to three days, and others had spam emails sent from their account.
Customers were advised to change their password, but some complained they were unable to do so.
In a post to its website, Yahoo! said a bug was affecting Yahoo! Mail and blamed it on a hardware problem that was bigger than originally thought.
- Rebecca Quilliam of APNZ