Optus will pay $8.8 million back to 235,000 customers it overcharged over a two year period from July 2011.
The telco today said it had identified a software coding error with its Optus SurePage answering service which meant 235,000 postpaid mobile and small business customers had been overcharged for the service from July 2011 to September this year.
An Optus spokesperson said a programmer had copy-pasted a code when programming, meaning 235,000 customers were placed on the SurePage service and provisioned and billed for it.
"It took us a while to work it out. [Customers] would call up and we would compensate that individual," the spokesperson told iTnews.
"The calls were starting to increase, and we looked at the IT behind it and realised there was a systemic issue."
Not all customers noticed they had been overcharged, the spokesperson said, given the relatively small nature of the majority of refunds. Over 100,000 customers would receive a refund of $10 or less, she said.
SurePage charges about $1.20 per call.
Optus said the issue had been fixed and from this month it had started refunding affected customers, with interest.
It said it had informed regulatory and competition bodies the Australian Competition and Consumer Commission (ACCC), the Australian Communications and Media Authority (ACMA) and the Telecommunications Industry Ombudsman (TIO) of the issue before acting.
Optus rival telco Telstra also recently dobbed itself in to the ACMA after it discovered it had overcharged more than 260,000 customers between 2006 and 2012 for international data roaming.
It was forced to repay $30 million to affected customers.
Telstra blamed incorrect information received from international carriers and its data clearing house, resulting in multiple flagfall fees being charged per session.