Telstra has received a formal warning from the Australian Communications and Media Authority (ACMA) for incorrectly overcharging $30 million in global roaming fees to 260,000 customers.

The incident related to the telco incorrectly billing customers over international data roaming charges for six years between 2006 and 2012.

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The ACMA said incorrect information was given to Telstra from international carriers and its data clearing clearing house.

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"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. Whether Telstra could rely on this exception was a key issue for this investigation," the ACMA said in a statement.

"The ACMA investigation found that Telstra could rely on this exception only until early 2009. It was then that Telstra first received a complaint from a consumer who had been incorrectly billed for multiple overseas data sessions."

The ACMA found Telstra failed to investigate and identify problems with its contractor after 2009 and breached the Telecommunications Consumer Protection Code.

The ACMA said it issued a formal warning because once the problem was discovered, Telstra identified all the customers who had been incorrectly charged and provided rebates. It said the telco was also not the original cause of the problem and Telstra reported the matter to the ACMA.

"Once we identified [the problem] we put in place immediate steps to prevent it happening again. We also proactively notified the ACMA and began a process to fully refund customers and to apologise for this happening in the first place," a Telstra spokesman said.

"We have permanently removed the charge in question and we accept this finding from [the] ACMA."

The TCP code was introduced in September last year. The ACMA said in the next quarter it would focus on the telco requirement to notify customers when they have used 50 per cent, 85 per cent and 100 per cent of their plan's value.