In a statement to the ASX, energy giant Origin said it was working to understand how many of its almost five million customers were affected by the privacy breach, a day after it alerted the public.
Names, addresses, dates of birth, phone numbers and account information could all have been viewed in the breach, Origin said.
Also at risk were the last four digits of credit cards, or the last three digits of bank accounts.
Origin chief executive Frank Calabria apologised to customers and said the company was working to contact those affected.
The Sydney-based provider has set up a contact number and "additional resources to help manage our response to this incident".
"One of our key priorities is taking action to secure our systems and ensure no further unauthorised access," Mr Calabria said.
"We are working with independent cyber experts to support Origin, and that work is continuing alongside the work of authorities."
The company continued to engage with federal cyber and information agencies on the incident as well as police, according to the statement.
Origin has 4.8 million customer accounts in Australia and provides electricity, natural gas, LPG, and internet services to residential and business customers.
8°C