The Australian Communications and Media Authority has issued a formal warning to MVNO Lycamobile for over-billing customers for more than two and a half years.
Lycamobile reported the billing issue to the ACMA itself, revealing that it charged customers $13,387 more than it should have between January 2011 and 31 August 2013.
The ACMA said that Lycamobile had conducted its own investigation of the issue and acted to temporarily fix the issue before implementing a permanent fix to its billing system.
The MVNO's billing system had on occasion erroneously treated fixed line numbers beginning with '4' as mobile phone numbers, according to the ACMA.
A statement issued by the ACMA today said that 12,596 Lycamobile customers were affected by the problem.
In February the Deputy Commissioner of Taxation made an application for the winding up of Lycamobile; however, Australian Securities and Investments Commission records show the application was dismissed last month.
The ACMA said that Lycamobile was the fourth telco the watchdog organisation has found to have breached TCP Code it came into effect in late 2012.
In February this year, the ACMA issued MVNO Live Connected with a warning for two breaches of the TCP code.
An attempt to contact Lycamobile for comment has been made.