A hardware fault at a Yahoo datacentre has prevented some YahooXtra customers from accessing their emails since Tuesday night, Telecom says.
Telecom spokeswoman Lucy Fullarton said the fault was affecting other Yahoo customers, not just YahooXtra or New Zealand email users.
It was not related to the malware attacks that resulted in many YahooXtra email accounts being hijacked and used to send out spam last week, she said.
Wellingtonian Jane Martin said she had been unable to check her YahooXtra email account for two days.
Telecom staff had been unsympathetic and unwilling to discuss compensation, she said.
Vice-president Jeffrey Bonforte said on Yahoo's official Tumblr blog the tech giant had "dozens of people working around the clock" to fix the problem, but that had proved harder than expected.
Bonforte did not say how many users were affected by the email outage but said the fault began at 7.27pm Tuesday.
"The issue has been harder to fix than we originally expected," Bonforte said.
Fullarton said Telecom would "do a review of this incident, as we always do".
Yahoo Mail is the most popular web-based email product among desktop computer users in the United States, according to analytics firm comScore. Yahoo Mail ranks second worldwide, behind Google's Gmail and ahead of Microsoft's Outlook.com.
A redesigned version of Yahoo Mail, which the company introduced in October, has met with a mixed reaction by users. Many users have griped that the new version eliminated popular features such as "tabs".
A Yahoo spokeswoman said the company did not disclose the location of its datacentres for security reasons, but confirmed it was not in New Zealand.