An Australian Communications and Media Authority investigation found the telco continued to bill 994 customers after they had transferred their service to other providers.

The errors, totalling $788,609, occurred between February 2011 and March 2021 at which point the company self-reported the matter to the ACMA and independently committed to issuing refunds.

Optus told the regulator the issue occurred due to incorrect billing end-dates being entered into its system for customers transferring to other telcos.

ACMA chair Nerida O'Loughlin said billing accuracy is a major concern for consumers, including small business.

'Errors like these by Optus can have a big impact on small business," she said.

"Fifteen of these customers were charged more than $10,000 which can be a huge amount for a small business to lose."

Optus is Australia's second-largest telco after Telstra.

"It should have systems in place to ensure it complies with all relevant regulation," Ms O'Loughlin said.

"It is alarming and unacceptable that such a fundamental flaw went undetected in Optus' systems for so long."

ACMA is monitoring Optus to ensure all affected customers are refunded and action is taken to reduce the risk of future billing errors.

Any further non-compliance could lead to significant consequences, with penalties of up to $250,000 for failure to comply with the ACMA direction.

Optus has contacted all affected customers and to date repaid 98 per cent of the charges plus interest.

The company is also reporting back to the ACMA regularly on its refunding program.

Australian Associated Press