Origin Energy has revealed details of a data breach after an alleged hacker claimed to have stolen personal details from millions of customers on Wednesday.

In a statement released on Thursday afternoon, the major Australian energy provider confirmed there had been “unauthorised access and disclosure” to some of its customer’s personal information.

Origin says customers’ names, dates of birth, phone numbers, and account information were among the impacted data.

The last four digits of credit card numbers and bank accounts were also stolen.

It is not yet known how many customers were affected by the breach.

The hacker, identifying as John Doe, claims to have stolen the private data of 2 million Australian customers in an email to 7NEWS on Wednesday.

The hacker also claims to have contacted Origin’s security teams, board, and customer services departments about the breach but had not received a response.

“They’ve shown no interest in resolving it before the data goes public,” the hacker’s message said.

The hacker has also threatened to release the information within 14 days unless an agreement is reached with Origin, linking to a countdown clock.

“We are giving you two weeks to reply and negotiate regarding the matter (please check the timer),” the threat said.

“If you do not message us and the timer reaches zero, we will leak 2 millions of your loyal customer’s data that we have.”

Origin Energy has approximately 5.8 million customers across Australia using its gas, electricity, and internet services.

The energy giant initially claimed that customers’ credit card and bank details had not been affected in a statement on Wednesday.

Origin CEO Frank Calabria apologised for the incident, and confirmed the company is working with the national Cyber Security Centre, Australian Federal Police, and the Office of the Australian Information Commissioner.

“We are contacting affected customers, offering support and have set up a dedicated contact number and additional resources to help manage our response to this incident,” Mr Calabria said.

“One of our key priorities is taking action to secure our systems and ensure no further unauthorised access. We are working with independent cyber experts to support Origin, and that work is continuing alongside the work of authorities.”