Origin Energy says it is continuing to investigate after the personal information of about 900,000 current and former customers was accessed in a cyber breach it disclosed last week.

Chief executive Frank Calabria has apologised to customers, saying the utility group has completed the first phase of its investigation and is notifying people who were affected.

“We don’t take for granted the trust customers place in Origin and our safeguarding of their information.” Mr Calabria said.

Origin says supporting affected customers is its priority and it has expanded help services, including specialist identity and cyber support, longer contact centre hours and a dedicated hotline.

It is working with cybersecurity specialists to contain the incident and has taken steps to secure its systems.

Origin is also working with the Australian Cyber Security Centre, the National Office of Cyber Security and federal police, and has notified the Office of the Australian Information Commissioner.

The company says it first identified a potential security threat in early July but, based on information available at the time, did not consider it credible.

New information emerged on July 22 indicating a security incident may have occurred, prompting immediate action, market disclosure and customer notifications, it says.

Mr Calabria says a criminal investigation limits the amount of detail the company can release.

Origin has urged customers to be alert for phishing attempts and other scams, warning criminals may seek to exploit the incident by impersonating the company.