Telecom cancelled the passwords of about 60,000 Yahoo!Xtra email accounts on Saturday night to stop spam being sent from customers' accounts.
Customers unable to log on had to change their passwords through the Xtra/Yahoo website after answering security questions. Once they had access to their usual email account, they were greeted with an email from Telecom saying: "We have cancelled your current password and you will need to, or have had to, change your password immediately to use your email account."
Last night, customers attempting to change their passwords through the Xtra/Yahoo website were advised of delays of up to an hour.
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About 15,000 Telecom customers had earlier been told about the need to get a new password after a cyber attack last weekend.
Telecom head of communications Andrew Pirie said: "We've been educating people over the last week to try and change passwords, and tens of thousands [of customers] have, but out of our 450,000 customer base, we are aware as of last night there were still 60,000 accounts that were having this problem."
If customers did not change their passwords spam could be sent from their email account, he said.
"Anyone who had been sent an email from Telecom must change their password otherwise they will be locked out of the system."
Telecom call-centres were being swamped with queries, he said.
"People should go online to change their passwords because we're getting literally hundreds if not thousands of people trying to call the help desks to walk them through it."
This morning, Telecom CEO of Retail Chris Quin said extra workers had been employed to deal with demand.
"We're making progress, and it's great that the last numbers I saw nearly 35,000 or so customers had reset their passwords.
"We need everyone to get through those so that we can make sure their accounts are safe."
Chris Quin said some customers had been threatening to switch providers.
"We accept the frustration and the hassle with this, and people worrying about the security issue. It's not unnatural for them to express those fears."
- Otago Daily Times with Newstalk ZBBy Shawn McAvinue