One Telecom customer was getting 20 nuisance messages a minute.
Telecom's troubled email service is suffering a fresh spate of problems just days after the New Zealand telco announced it would stick with Australian-based Yahoo.
Yahoo!Xtra account holders contacted the Herald on Sunday complaining accounts had been hacked or clogged with unusually high levels of nuisance spam.
Corporate communications boss Lauren Young said her Yahoo!Xtra home email account was under siege. Over a 72-hour period, tens of thousands of emails flooded into her inbox, peaking at 20 messages a minute.
The Counties Manukau District Health Board communications manager rang the Telecom helpdesk and changed her password, but the unwanted spam continued through to yesterday.
"I've never experienced anything like this and it still keeps going," said Young. "I'm still under attack and have been since Thursday."
Telecom is playing down the latest attack, saying it isn't as severe as an incident in February where more than 80,000 Yahoo!Xtra customers had their accounts compromised.
Yesterday, Telecom spokeswoman Holly Linnell said numbers weren't available on those affected in this week's attack, nor how many accounts were locked or passwords needed changing.
"I understand from the tech team they could work on a case-by-case process to allow people who had looked like they had been compromised to be contacted," Linnell said.
Linnell apologised to customers affected by the latest incident - in particular those whose accounts had been misused to send suspicious emails, as well as those who have received such emails.
"It was extremely disappointing to us that this incident seems to have recurred," she said.
The telco was talking with Yahoo! to try to understand the cause.
Linnell said new processes had been implemented at Yahoo! and these were proving effective in containing spamming activity. The processes included automated messages to change passwords.
Systems were in place to redirect users of Xtra Mail to a website advising passwords be changed.
Other email client users would be blocked from entering their accounts and would need to change their passwords via the webpage or by calling the helpdesk.
In the meantime, Linnell warned users against clicking on links that looked suspicious, even if they came from friends or contacts.
Labour Party communication and IT spokeswoman Clare Curran said Telecom should be transparent with any problems.
"The public are on reasonably high alert at the moment and it's incumbent on Telecom to be upfront with their customers."
In the meantime an exasperated Young, who has spent hours trawling through her inbox to retrieve genuine messages, would like compensation.
"I would like them to apologise for not getting it right and not preventing this. I would also like them to compensate anyone who has suffered this by offering a refund on their account fees."