Some of Spark's Xtra users have been caught up in a massive data breach at its former email provider, Yahoo.
Hackers stole information from about 500 million Yahoo users in a 2014 cyber attack.
Yahoo has said the information did not include any credit card data, and it believed the attack was state-sponsored.
Spark said it included some of its users' information, including names, email addresses, dates of birth, hashed passwords and, in some cases, encrypted or unencrypted security questions and answers.
It was unsure how many of its Xtra email customers were affected, but said anyone who had not changed their password since late 2014 should do so immediately.
Spark said it would contact affected customers directly as soon as it knew who they were.
The scale of the hack eclipses other recent major tech breaches - such as MySpace (359 million), LinkedIn (159 million) and Adobe (152 million).
- RNZ / BBC