Internet provider Spark says it's still trying to work out whether its customers have been affected after 500 million Yahoo clients had information from their accounts stolen.
It comes just days after Spark announced it would be ditching Yahoo as the provider of its email service Xtra Mail.
Global internet giant Yahoo said at least 500 million user accounts had information taken in 2014 by what it believes is an attack by a "state-sponsored actor".
The data may include email addresses, telephone numbers, dates of birth and hashed passwords - but not credit card or bank account information, it said.
It said it was working with law enforcement on the issue.
In New Zealand, Spark, formerly Telecom, has for nine years run its email system - Xtra Mail - through Yahoo's servers.
In a statement, a spokeswoman said it was working with Yahoo to figure out whether any New Zealand customers had been affected.
"As a precaution, we encourage anyone who has not changed their password in the last 12 to 18 months to do so," she said.
Yahoo says it's notifying potentially affected users and taking steps to lock the hackers out.
Around 400,000 Xtra Mail customers had to change their passwords following what is believed to be a breach of the servers in 2013.
Spark last week announced New Zealand-based company SMX will be the new manager of its email services from early next year.