Spark is advising its Xtra customers to change their password and security questions following the revelation its email partner Yahoo was hacked in 2014.

The multinational tech company announced on Friday that some of its user account information was stolen from Yahoo's global network back in November 2014, which included some of Spark Xtra email customers' data.

File Photo.

Yahoo believes information associated to at least 500 million user accounts were stolen by what it describes as a 'state-sponsored actor”.

Some of the information may have included names, email addresses, telephone numbers, dates of birth, bycrpt-protected hashed passwords, plus encrypted or unencrypted security questions and answers.

'Spark will be communicating directly with customers who we believe may have been impacted as soon as we have more information,” says spokeswoman Michelle Baguley.

'Yahoo has no evidence that the stolen bcrypt-protected passwords or security questions and answers were used to gain unauthorized access to Spark accounts.”

Yahoo's investigation into the hack suggests the stolen information did not include unprotected passwords, payment card data, or bank account information. Payment card data and bank account information are not stored in the system that was affected by the hack.

Spark is now reminding all its customers to change their password and security questions for their Xtra account and any other account which they used the same or similar information.

'To maintain a secure online profile, Spark advises all Xtra users to regularly update account settings with a strong, difficult-to-predict password.

'All Xtra customers who have not changed their password or security questions since 2014, or are unsure if they have, should do so now on the Spark website.”

Michelle says Spark is currently in the process of preparing to move all of its email system back home to New Zealand, with a number of customers already receiving requests to register on the company's website.

'We thank those customers who have already registered and encourage those who have not registered, to do so. If you've already registered to have your email moved to SMX, you don't need to do that again – any changes you make to your password will be applied to the new system.”

To change your Xtra email password or security questions visit: www.spark.co.nz/changepassword