Australian carrier Optus has been formally warned by telecoms regulator the Australian Communications and Media Authority (ACMA) for inaccurately billing over 237,500 customers prior to September 2012.

The regulator found that between November 23 2008 and September 30 2012, Optus overcharged some customers of its SurePage and SpinVox services, which are additional messaging plans for postpaid users.

The authority revealed that errors in implementing an IT upgrade caused 2,600 customers to be overcharged between November 2008 and July 2011. Then, between July 1 2011 and September 30 2012  an additional 235,000 customers were overcharged by approximately AU$8.9m.

although the error was made by an IT programmer during a project upgrade, ACMA also found that an oversight occurred in quality control of the software code for the upgrade. Optus’ SurePage product manager became aware of the problem in late August 2011 but the root cause of the problem was not identified until July 2012, and a permanent fix to address the root cause was not implemented until September 30 2012, according to ACMA.

A warning was issued to Optus by ACMA, stating that the operator had failed to identify and fix the root cause of the billing inaccuracy as soon as it could. However, the regulator noted that Optus reported the matter itself and appears to be otherwise compliant with the revised Telecommunications Consumer Protections Code (the TCP Code) 2012. ACMA also took into account that Optus had proactively implemented a comprehensive programme to compensate affected customers.

“I want to note Optus’s constructive engagement with the ACMA’s investigation,” said ACMA chairman Chris Chapman. “We are now confident that Optus has rectified the billing inaccuracy and is reimbursing with interest all affected customers.”

An Optus spokesman said that the firm recognises ACMA’s formal warning and once again apologises to affected customers.

“We have been working with the ACMA since October 2013 to proactively inform them of this billing error,” he said. “We also issued a press release explaining the mistake and compensation measures we were taking.”

The spokesman added that Optus has a “single-minded focus to make things better for our customers”, which includes being transparent about mistakes, fixing them and compensating customers.

“The majority of affected customers have already received compensation, with the rest receiving it by the end of March,” he added.

Earlier this month, rival Telstra was fined AU$10,200 by ACMA for leaking personal details of around 16,000 of its customers. The operator made spreadsheets containing the details accessible via Google Search between February 2012 and May 2013 and has since undertaken to cease use of the Oracle Right Now SaaS CRM system, although it was not clear whether this was a voluntary decision or a requirement of the Australian regulatory authorities.

Since Optus and Telstra’s errors were identified, ACMA noted that customer complaints have gradually fallen. This month it announced a “sustained reduction” in complaints to the Telecommunications Industry Ombudsman (TIO) from consumers regarding customer service issues.

It said that data published by the TIO this month reveals that in the quarter ended December 2013, the body received the lowest amount of complaints in a quarter in six years.