Telstra will refund up to $A30 million to customers in Australia after the telecommunications giant overcharged for global roaming services on mobile phones since 2006.
Australia's largest telco identified the problem following an audit earlier in 2012, and most of Telstra's affected customers have now been refunded.
"Telstra became aware of an issue whereby some customers were charged multiple data session fees due to the way international carriers generate their data usage records," a Telstra spokesman said in a statement on Friday.
"Once we identified the issue, we put immediate steps in place to prevent further multiple charging."
The spokesman said overseas carriers had presented the charges of Telstra customers multiple times.
The communications watchdog said it would investigate whether Telstra had breached the billing provisions of the Telecommunications Consumer Protection Codes.
The Australian Communications and Media Authority (ACMA) said said it would work with the Australian Competition and Consumer Commission (ACCC) and Telstra to ensure all affected customers would receive refunds.