The Australian Communications and Media Authority (ACMA) has found Optus in breach of the Telecommunications Consumer Protections (TCP) Code by charging businesses for landline services they did not receive between February 2011 and March 2021, as the businesses had shifted to other telcos.
Optus self-reported the breaches, saying the issue was incorrect billing end dates being entered into its systems, and that it would be issuing refunds.
ACMA chair Nerida O'Loughlin said it was alarming that such a "fundamental flaw" went unnoticed for so long.
"Errors like these by Optus can have a big impact on small business. Fifteen of these customers were charged more than AU$10,000 which can be a huge amount for a small business to lose," the chair said.
"Optus is the second-largest telecommunications company in Australia and should have systems in place to ensure it complies with all relevant regulation."
ACMA has issued Optus with a formal direction to be compliant with the TCP.
The regulator said it would be keeping a close eye to ensure refunds are made and the error does not reappear. However, at this stage, Optus has contacted all 994 affected customers and refunded 98% of the charges as well as paid interest.
Earlier in the week after undertaking a review, Optus found it underpaid the wages of some of its retail staff between 1 January 2014 to 17 March 2020 for workers covered by the Optus retail enterprise bargaining agreement.
An Optus spokesperson said the payment gap was due to a "range of changes" on the telco's processes and systems in recent years.
"The majority of Optus retail employees included in the review will receive AU$500 or less, and in many cases, nothing at all. Importantly, Optus is making changes to its processes to prevent future payroll adjustments," they said.
"Optus has commenced contacting current and former employees to apologise for the error."
The Communication Workers Union said on Monday the average underpayment amount from the review was AU$757, with interest and superannuation to be paid on top.