The Australian Communications and Media Authority (ACMA) has issued a warning, but no monetary punishment after it found Optus had overcharged mobile customers around $9 million.

A recent investigations by the communications watchdog revealed Optus messed up the bills for thousands customers for the messaging services SurePage and SpinVox between November 2008 and September 2012.

The company took an extra $8.9 million from around 237,500, but says it will reimburse them.

ACMA says Optus breached the Telecommunications Consumer Protections Code by failing to identify and fix the problem within an adequate timeframe.

Reports say the excess charges were the result of a technical error, which added charges to customer accounts.

ACMA chairman Chris Chapman says Optus acted “constructively” in its subsequent dealing with investigators.

“We are now confident that Optus has rectified the billing inaccuracy and is reimbursing with interest all affected customers,” Mr Chapman said.

Optus did take the initiative to report the problems itself, and fully admitted breaching the code.