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.
Elise Davidson from the Australian Communications Consumer Action Network (ACCAN) says hundreds of thousands of people are potentially affected by the billing mistake.
"When customers travel overseas they're obviously roaming on international networks," she said.
"Those providers overseas then send the information to a clearinghouse that's also offshore, which then processes all that information about data sessions and sends it back to Telstra in Australia.
"What's happened is the way that the international carriers have supplied the information to the clearinghouse has broken up what could be quite large, long data sessions into smaller sessions." She says customers are charged a fee of around 50 cents at the start of each data session.
"So you may have been charged that fee, say five times over a 10 minute session where you just have had one 10 minute session," she said.
Refunds provided 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.
Ms Davidson says any of Telstra's former customers who have been overseas in the past six years should get in contact with the company.
"For existing customers, you probably will have been contacted, if not, probably you will be in the next week or so," she said.
"If you haven't heard from Telstra during that time, you should then get in touch with them.
"It is our understanding however that Telstra have found out about this problem a little earlier this year and so they stopped charging customers for this particular charge that is in question.
"So if you travelled say only in the last six months, it may not apply to you so you may not hear from them in that regard." ACMA investigation Telstra says it also came forward to the Australian Competition and Consumer Commission (ACCC) and the Australian Communications and Media Authority (ACMA) when it realised it had been overcharging.
ACMA has now launched its own investigation.
"Following notification by Telstra of matters relating to overcharging, the Australian Communications and Media Authority has commenced an investigation into whether Telstra has breached the billing provisions of the Telecommunications Consumer Protection Codes," ACMA said in a statement.
"The ACMA is working with the Australian Competition and Consumer Commission and Telstra to ensure an appropriate outcome for all affected customers, including possible refunds.
"As this is a current investigation, the ACMA will not be making further comment." Both Optus and Vodafone say their customers have not been overcharged because they do not bill the way Telstra does.