Yahoo says that a 2013 breach affected all 3 billion of its accounts, tripling the number originally reported.
Yahoo last year said it believes "state-sponsored" hackers stole data from more than one billion user accounts in what was thought to be the largest data breach at an email provider.
Yahoo today revealed that all 3 billion of its accounts were impacted, three times what was originally reported.
The hack happened in 2013 but was only revealed last year.
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Spark - which cuts its ties with Yahoo last year - said every Xtra email that existed in 2013 was impacted by the data breach.
"While this is not a new security breach, it's obviously disappointing to learn that Yahoo have exposed their entire customer base, including Spark Xtra customers, to this risk," Spark said.
Spark dropped Yahoo last year as its service provider for Xtra Mail after issues with security, phishing and spam, and migrated the operation back to New Zealand.
The hack is believed to be separate to one which exposed 500 millon Yahoo accounts, which compromised up to 130,000 Spark customer emailed addresses.
- Additional reporting: Associated Press.