Australian regulator ACMA has formally warned Optus for a billing error that resulted in over 237,000 mobile customers being overcharged to the tune of A$8.9 million ($8.2 million).
An IT error resulted in customers being incorrectly billed for two post-paid messaging services between September 2008 and September 2012, ACMA said.
The investigation found that the product manager for one of the messaging services discovered the problem in late August 2011, but the root cause wasn't discovered until July 2012, and a fix was not implemented until September 2012.
Under Australia's Telecommunications Consumer Protection Code, operators are required to verify the accuracy of their billing systems and identify and rectify systemic problems as soon as practically possible.
The regulator said the decision to warn but not penalize Optus took into account the fact that Optus self-reported the error, is otherwise generally compliant with the consumer code, and has implemented a program to repay affected customers with interest.
In September 2013, ACMA warned Optus rival Telstra for inadvertently overcharging international data roaming customers. In this case, the problem was due to inaccurate information provided by international operators, and Telstra voluntarily compensated affected users.