TELSTRA is refunding customers about $30 million after making the embarrassing admission it has overcharged customers for global roaming services on mobile phones since 2006.

The telephone company has been writing to customers for the past month informing them their data charges while travelling overseas had been ''incorrectly calculated'' and they would be given refunds.

Despite overcharging going back to 2006 and involving tens of millions of dollars, Telstra said it only became aware of the issue when it conducted an audit earlier this year.

Complaints about global roaming charges have dogged telecommunications companies for years.

The Telecommunications Industry Ombudsman, an industry-funded dispute handling body, reported a consumer who bought a $129 monthly plan so she could make calls during a nine-week holiday in Europe, returned to a $75,000 bill that subsequently increased to $147,908.