Telecom says more of its 450,000 YahooXtra account-holders had their accounts compromised by hackers overnight.

Spokeswoman Lucy Fullarton refused to estimate how many customers had had their accounts hijacked since the new wave of attacks began over the weekend.

However, the volume of malware-infested spam being sent from YahooXtra accounts suggests it is likely to have been in the tens of thousands.

Telecom's email outsourcer, Yahoo, has been locking people out of their accounts, once it has detected their accounts are compromised, until customers change their passwords.

Fullarton said it was continuing to investigate the cause of the issue.

Telecom outsourced its email service to Yahoo in 2007 and reviewed the email service earlier this year after 87,000 accounts were compromised in a February attack.

As a result of the review, Telecom decided to move customers from a troubled "bespoke" system operated by Yahoo, on to Yahoo's main email platform. Telecom retail chief executive Chris Quin said in April that he was confident that would make the service more reliable.

Telecom said in September that the migration had begun and would take a few months.

However, Telecom and Yahoo have both refused to provide any assurances that this week's problems have not affected customers on the new platform and have instead denied that the migration was intended to prevent such attacks. Fullarton said cyber-crime was a "global issue".