Telstra has begun refunding customers around $30 million after overcharging on global data roaming services since 2006.
The company says it became aware of an issue whereby some customers were charged multiple data session fees due to the way international carriers generate their records.
Telstra says once it was alerted, it put immediate steps in place to stop more customers being affected.
It says it is now in the process of contacting affected customers and refunding them.
"We are satisfied that we have identified every affected data session and are in the process of contacting affected customers to provide a full refund, with the majority now complete," Telstra said in a statement.