New Zealand internet provider Spark says some of its customers have been affected by a global breach at Yahoo.
Global internet giant Yahoo said earlier on Friday at least 500 million user accounts had been hacked in 2014.
It was working with law enforcement on the issue.
In New Zealand, Spark, formerly Telecom, has run its email system - Xtra Mail - through Yahoo's servers for nine years.
Spark says Yahoo has confirmed information from some of its Xtra customers is included in the stolen data.
Yahoo has no evidence the stolen bcrypt-protected passwords or security questions and answers were used to gain unauthorised access to Spark accounts.
But the stolen account information may have included names, email addresses, telephone numbers, dates of birth and hashed passwords and, in some cases, encrypted or unencrypted security questions and answers.
The stolen information did not include unprotected passwords.
Spark says it will get in touch directly with affected customers when it has more information.
All customers should change their password and security questions for their Xtra account and any other account on which they have used the same or similar information.
The revelation comes just days after Spark announced it would be ditching Yahoo as the provider of its email service.
New Zealand-based company SMX will be the new manager of Spark's email services from early next year.