YahooXtra customers have been hit with another wave of spam attacks, forcing Telecom to lock down users' email accounts.
It's unknown how many are affected, but an attack in February compromised around 87,000 accounts.
Telecom spokesman Richard Llewellyn said customers would be locked out until they changed their password.
"If Yahoo identifies an account as having been compromised, they will block the account until the customer resets their password.''
Mr Llewellyn said the company became aware Yahoo had been experiencing spam-related issues yesterday morning.