Australia's communication authority has issued a formal warning to leading mobile service operator Telstra for incorrect billing of a large number of international data roaming customers.

The warning comes, after Telstra self-reported to the Australian Communications and Media Authority (ACMA) in November 2012, that it had overcharged over 260,000 customers a total of around $30 million for international data roaming.

The decision was limited to issuing a warning as ACMA took into account the fact that Telstra was not the original cause of the problem. It also found that, this was the first time a billing issue of this nature had been investigated under the TCP Code. Further, to the company's advantage was the fact that, Telstra itself had reported the matter and had proactively implemented a comprehensive program of compensation to mitigate the harm for affected customers. ACMA also found that Telstra to be otherwise compliant with the relevant parts of the TCP Code 2012.

A press release issued by ACMA on Monday said the overcharging occurred over a 6 year period.

Between 2006 and 2012 Telstra incorrectly billed customers multiple flag fall fees for single data sessions. The company said, the incorrect bills reflected wrong information Telstra received from international carriers and its contracted data clearing house. Most customers were overcharged small amounts, the release said.

ACMA issued the warning to Telstra for breaching the Telecommunications Consumer Protection Code 2007 (TCP Code), the release said.

The TCP Code requires providers to bill customers accurately allowing for certain exceptions. One such exception is where the inaccuracy is caused by reliance on information provided by contractors. The key issue of the investigation was whether Telstra could rely on this exception.

The ACMA investigation revealed that, Telstra could rely on this exception only until early 2009. ACMA found that by then the company had received the consumer complaint informing them of the incorrect billing for multiple overseas data sessions.

ACMA says that billing inaccuracies, after that complaint was received, were caused by Telstra's failure to investigate and identify the problems.

'Accurate billing is of the utmost importance' said ACMA Chairman, Chris Chapman.

'Our investigation makes it very clear that all telcos need to listen to their customers who report billing problems and be vigilant about any potential issues with the information provided to them by third parties,' Chapman said.

The release said that once the problem was discovered, Telstra promptly identified all customers who were incorrectly charged for international data roaming and is systematically providing rebates. It has also permanently ceased charging a flag fall fee for international roaming data services.