Medibank Private won't be able to provide annual tax statements on time to the majority of its customers because of a problem upgrading its technology.
The health insurer said on Thursday most customers will have to wait until after July 15 for their statements, but didn't give a new date for delivery.
Medibank chief executive Craig Drummond said the situation was unacceptable for customers.
"I sincerely apologise to our customers for this disruption. It is important for customers to know their personal data has not been compromised," Mr Drummond said.
"Our team is working around the clock to resolve this issue, and I want to assure customers that our focus is on minimising the impact this will have on them."
Medibank, which became aware of the problem on Wednesday, said it will post further information on its website and contact customers directly when it has further information.
However, customers who lose out financially because of the delay - which doesn't affect Medibank subsidiary AHM - will have to contact the insurer to make a claim.
