“Normal business operations have resumed, and necessary action will continue to be taken to further safeguard the IT environment including customer data,” it said.

The breach of Medibank’s systems comes as Optus continues to deal with the fallout from the cyberattack that compromised the personal data of millions of Australians last month.

The company said the Australian Cyber Security Centre (the Australian Government lead agency) was engaged and it is working with them to keep national response agencies updated and to receive information and intelligence which is assisting with the resolution of the incident.

Medibank said the incident will not disrupt the momentum of the business and confirmed that it is still on track with the guidance provided at its full-year results in August.

Medibank shares, which had been suspended since the incident was made public last Thursday, dropped 5 per cent in early trade to a low of $3.35 this morning.