Telecom has cancelled the passwords of about 60,000 Yahoo! Xtra email accounts it believes may have been compromised.
They join another 15,000 customers Telecom has been contacting to change passwords after a cyber attack last weekend where hackers sent infected messages to third parties.
Telecom says all those customers will be required to enter a new password the next time they log into their email account.
Retail chief executive Chris Quin says Telecom was told by Yahoo! on Saturday evening about the further 60,000 accounts at risk.
He says Telecom then moved quickly to ensure those at risk changed their passwords as soon as possible.
"We're trying to get hold of all the accounts that it has affected and have those passwords changed - as soon as the password's changed, everything that's sent through that account will then be safe."
Mr Quin says people using other devices, such as smartphones or tablets, will need to update them with their new account passwords.
A Telecom spokesperson says changing passwords from a locked account is quite simple and account holders will get a message to do so next time they log in.