Health insurer Medibank Private says it has been hit by a cyber attack.
The company said "unusual activity" had been detected on its network on Wednesday, but there was no evidence sensitive data, including customer information, had been accessed.
Some customer-facing systems have been taken down which will cause "regrettable disruptions" to some customers, but health services will still be available, Medibank said.
It is the latest cyber attack after the Optus breach last month, which affected millions of customers.
Chief executive David Koczkar said Medibank recognised its responsibility, given the sensitive data it holds about people's health.
"I apologise and acknowledge that in the current environment this news may make people concerned," he said.
"Our highest priority is resolving this matter as transparently and quickly as possible."
Medibank's ahm and international student policy management systems are currently offline.
Cybersecurity expert and founder of StickmanCyber Ajay Unni said insurers such as Medibank were valuable targets for malicious cyber activity, thanks to the highly sensitive personal information that they held.
"It is encouraging to see large organisations like Medibank now coming forward and taking accountability for cyber attacks and data breaches," he said.
"Communication is key in any incident, including cyber, or the public will speculate and draw their own conclusions, leading to erroneous information being circulated.
"Being on the front foot and taking action, even when it may be disruptive to business, along with keeping customers and the public up-to-date is a step in the right direction."
Mr Unni said businesses, regardless of their size and scale, needed to at least inform the Office of Australian Information Commission of a suspected or confirmed breach, and request assistance from Australian Cyber Security Centre and any third parties who could help with the investigation and remediation.
Medibank said investigations into the cyber attack are ongoing.
The company requested a trading halt on the ASX while it investigated.
