60,000 people with Yahoo!Xtra email accounts are having their passwords cancelled, after last weekend's cyber-attack.
They are in addition to the 15,000 customers Telecom has been trying to reach over the past few days.
Users will have to put in new password information when they log on to their email accounts.
Telecom's retail chief Chris Quin said the best way to protect customers was to cancel the current passwords of the additional accounts.
He said the matter was being taken very seriously.